Executive Summary: Key Legal and Evidentiary Issues

• Reasonableness of the arbitrator’s interpretation of Article 35.11 of the collective agreement as setting only the process for recovery of overpayments, not a freestanding entitlement to recover.
• Use of estoppel in the labour arbitration context to bar Canada Post from recouping wage overpayments made by mistake of fact, despite the absence of individualized evidence of reliance by each employee.
• Interaction between common law restitution principles on mistaken payments and the specialized, flexible approach of labour arbitrators to equitable doctrines.
• Significance of the omission of Supreme Court authority (B.M.P. Global) from the arbitral reasons, and whether that omission rendered the award unreasonable.
• Attempt by Canada Post to raise unjust enrichment for the first time on judicial review, and the court’s insistence that new issues of this nature should generally be addressed at first instance before the specialized tribunal.
• Scope of judicial deference on judicial review of labour arbitration awards, applying Vavilov’s reasonableness framework and emphasizing arbitral expertise in collective agreement interpretation.

Background and facts

Canada Post Corporation (CPC) and the Canadian Postmasters and Assistants Association (the Union) were parties to a collective agreement governing compensation for employees working in various classifications, including Postmaster and Senior Assistant roles. A recurring issue arose where employees were assigned to “acting” positions at higher classifications rather than being formally appointed into those higher roles. The parties disagreed about how incremental pay increases should be calculated for these acting employees. The Union took the position that employees acting in Postmaster or Senior Assistant roles should receive pay increments “as if appointed” to those positions, meaning their compensation should reflect the full incremental schedule of the higher classification. CPC, by contrast, maintained that its existing method of calculating pay for acting assignments was correct under the collective agreement, and it continued applying that interpretation after the Union filed a national policy grievance in 2016. During this time, CPC did not notify affected employees that there was any risk that amounts paid to them while acting in higher positions might later be treated as overpayments and recovered. Discussions about the pay issue, and the possibility of overpayments, took place at the national level and between counsel, but there was no direct warning to individual employees.

The first arbitration award and its consequences

The first arbitration award by Arbitrator Wayne Thistle was issued in 2021 and is not under judicial review in this court decision. In that award, the arbitrator determined the proper interpretation of the collective agreement in relation to pay for acting positions and ruled on how employees should be compensated when acting in higher classifications. Following this 2021 ruling, CPC implemented the arbitrator’s interpretation going forward. Once the proper interpretation was applied retroactively, it became clear that some Union members had been underpaid before 2021, while others had been overpaid under CPC’s earlier approach. Underpayments were dealt with: employees who had received less than they were entitled to under the arbitrator’s interpretation were compensated. Overpayments, however, became the focus of the second arbitration. The question was whether, and to what extent, CPC could recover overpayments made over a period that stretched back roughly eight years and covered about five years of actual overpayment, even though the parties had not provided detailed evidence of the precise amounts involved.

The second arbitration: overpayments and mistake of fact

The second arbitration (the Award that is the subject of the judicial review) proceeded largely on an agreed statement of facts; no witnesses were called by either side. Arbitrator Thistle considered the legal framework for recovery of overpayments, relying heavily on established arbitral and common law authorities, in particular the 2016 decision in Re Ottawa Board of Education and Federation of Women Teachers’ Associations. That decision, in turn, drew on English common law and Supreme Court of Canada jurisprudence concerning repayment of monies paid under a mistake of fact. Applying these authorities, the arbitrator held that the overpayments at issue arose from a mistake of fact, not a mistake of law. That distinction can be important in restitution law, because payments made under a mistake of fact are generally recoverable subject to defences, whereas payments under a pure mistake of law historically raised different considerations. In this case, the finding that CPC’s error was one of fact rather than law was not challenged on judicial review. The arbitrator recognized two main defences that could preclude recovery of an overpayment made under a mistake of fact: (1) a material change of position, and (2) estoppel. He focused particularly on estoppel in light of how long CPC’s pay practice had continued, the absence of notice to employees, and the practical hardships that arise when individuals plan their finances based on income that is later characterized as an overpayment.

Policy terms and collective agreement provisions

A central contractual provision in dispute was Article 35.11 of the collective agreement, under the heading “Recovery of Overpayments.” This clause stated that when an employee is overpaid through no fault of their own and the overpayment exceeds $50, the paying office must, before taking recovery action, advise the employee of the intention to recover; if the employee indicates that the proposed recovery will create hardship, the recovery is to be limited to 10% of the employee’s pay each period until repaid, with full recovery permitted from final pay if employment ends. CPC argued this clause did more than set out procedure; it said Article 35.11 both recognized and conferred a clear entitlement on CPC to recover overpayments as a matter of contractual right, with the article then prescribing the mechanics of recovery. The arbitrator, however, interpreted Article 35.11 as governing only the process to be followed where recovery is otherwise permissible, and not as creating or confirming a substantive entitlement to recover in every situation. In his view, the right to recover overpaid sums still depended on broader legal principles, including mistake-of-fact restitution and available equitable defences such as estoppel. After reviewing the collective agreement, relevant legislation, and case law, the arbitrator concluded that CPC had violated the collective agreement and that the affected employees were not compelled to repay any compensation arising from the identified overpayments in this particular context, given the established defence.

The judicial review application and standard of review

CPC applied to the Ontario Divisional Court for judicial review of the second arbitration award. It raised three main issues: (1) that the arbitrator had failed to reasonably interpret Article 35.11 and had improperly treated it as purely procedural; (2) that the finding of estoppel, which blocked CPC’s recovery claim, was unreasonable; and (3) that the award was unreasonable because it did not discuss unjust enrichment as an independent restitutionary basis for recovery. There was no dispute that the applicable standard of review was reasonableness, under the Supreme Court of Canada’s framework in Canada (Minister of Citizenship and Immigration) v. Vavilov. The court emphasized that the hallmarks of a reasonable decision are justification, transparency, and intelligibility, and that reviewing courts must exercise restraint and respect the specialized role and expertise of labour arbitrators, particularly in the interpretation of collective agreements and the application of equitable doctrines in labour relations.

Court’s analysis of the collective agreement and estoppel

On the interpretation of Article 35.11, the court held that the arbitrator had clearly considered the collective agreement, including the specific provision relied upon by CPC. The question for the court was not whether a different reading of the article could also be plausible, but whether the arbitrator’s interpretation fell within the range of reasonable outcomes. The Divisional Court found that it did. Interpreting Article 35.11 as setting out a process for recovering overpayments—rather than a blanket entitlement—was consistent with the arbitrator’s broader analysis that overpayments made under a mistake of fact are generally recoverable only where defences such as estoppel do not apply. The court rejected CPC’s argument that this reading rendered Article 35.11 meaningless, explaining that the clause still had work to do in cases where no defence was established; in those scenarios, the article would govern the mechanics of recovery. The court then turned to the estoppel finding. CPC argued that the arbitrator had erred by not expressly working through the three-part estoppel test and by not identifying evidence of actual reliance or detrimental change of position on the part of individual employees. The Divisional Court noted that, under Supreme Court authority in Nor-Man Regional Health Authority, labour arbitrators are not strictly bound to apply equitable doctrines in the same technical manner as courts of law. They have a broad mandate to adapt equitable principles to the realities of labour relations, provided they act reasonably and consistently with the statutory scheme and the collective bargaining context. In that light, the court held that the absence of a formal, step-by-step estoppel analysis did not, by itself, render the award unreasonable. The arbitrator’s reasons, read as a whole, showed that CPC had overpaid wages over a lengthy period, that the employees were not at fault, that CPC did not warn them that recovery might be sought, and that there is recognized hardship when people are led to believe their income is higher than it truly is. Those findings, combined with the arbitrator’s reliance on established arbitral jurisprudence, provided a rational basis for concluding that estoppel applied to bar CPC from recovering the overpayments in this particular case, without requiring evidence of individualized detrimental reliance from each affected employee.

Supreme Court authority on mistaken payments and unjust enrichment arguments

The Divisional Court also addressed a potential concern related to the Supreme Court’s decision in B.M.P. Global Distribution Inc. v. Bank of Nova Scotia, a leading case on repayment of monies paid under a mistake of fact, particularly in banking and financial contexts. B.M.P. had been cited in earlier jurisprudence on mistaken payments, but it had not been expressly put before the arbitrator or considered in the award. Both parties agreed that, while B.M.P. has been influential in civil cases—especially involving financial institutions—it has not yet been widely integrated into labour arbitration decisions. CPC suggested that the omission of B.M.P. undermined the award’s reasonableness; however, the Union emphasized that B.M.P. arose from a different factual and legal context (a fraudulent cheque scenario involving a bank and a corporate customer), and that labour arbitrators enjoy flexibility in how they incorporate or distinguish general restitution principles. The court agreed that the omission of B.M.P. did not render the award unreasonable or incorrect. It reiterated that labour arbitrators are not strictly bound to apply equitable doctrines, including restitution and estoppel, in the same manner as civil courts, provided their approach is justified and coherent in the labour relations setting. On unjust enrichment, CPC attempted to argue on judicial review that, even if estoppel applied, the arbitrator ought to have considered whether CPC had a claim for repayment based on unjust enrichment. The Divisional Court rejected this argument because unjust enrichment had not been raised before the arbitrator. Relying on Supreme Court guidance in Alberta (Information and Privacy Commissioner) v. Alberta Teachers' Association, the court stressed that new issues engaging a tribunal’s specialized expertise should generally be presented and argued before the tribunal in the first instance. Allowing CPC to introduce unjust enrichment for the first time on judicial review would deprive the court of the benefit of the arbitrator’s views and would be contrary to the proper role of a reviewing court.

Outcome and monetary consequences

Ultimately, the Divisional Court concluded that the arbitration award met the Vavilov standard of reasonableness. The arbitrator’s treatment of Article 35.11, his adaptation of the doctrine of estoppel in the labour context, his reliance on established arbitral and common law authorities, and his handling of the evidentiary record all demonstrated justification, transparency, and intelligibility. The award fell squarely within the area of labour arbitration expertise, and CPC did not discharge its burden of showing that it was unreasonable. As a result, the application for judicial review was dismissed, and the arbitration award in favour of the Union remained in force. The successful party in the court proceeding was the Canadian Postmasters and Assistants Association. The court ordered Canada Post Corporation to pay the Union costs in the agreed-all-inclusive amount of $5,000. There were no additional quantified damages or monetary awards specified beyond this costs order, and no separate global figure was set out in the decision for any underlying overpayments or repayments.