“Most of the case law on this subject has arisen in the context of applications to increase child support retroactively. The general principles and underlying rationale apply, however, to cases where the paying spouse seeks to retroactively reduce child support payable to reduce or eliminate arrears that have accrued or to obtain reimbursement of an overpayment.
In deciding whether to rescind child support arrears, the court will consider, inter alia, the following factors:
a) The nature of the obligation to provide support, whether contractual, statutory or judicial;
b) The ongoing financial capacity of the payor spouse;
c) The ongoing need of the custodial parent and the dependent child;
d) Unreasonable and unexplained delay on the part of the parent seeking to enforce payment of the obligation;
e) Unreasonable and unexplained delay on the part of the payor spouse to seek appropriate relief from his or her obligation;
f) Where payment of the arrears will cause undue hardship, the exercise of the court’s discretion to grant a measure of relief is based on a holistic view which weighs the needs of the child and custodial parent, and the current and future capacity of the parent who must pay the arrears.
(See DiFranscesco v. Couto, 2001 CanLII 8613 (ON CA), [2001] O.J. No. 4307 (ON C.A.) at para 23.)”