What authority does the court have to vary custody and access orders under s. 17 of the Divorce Act?

The leading authority on the exercise of the court's discretion to vary custody and access orders under s. 17 of the Divorce Act is Gordon v. Goertz, 1996 CanLII 191 (SCC), [1996] 2 S.C.R. 27. There, Madam Justice McLachlin, as she then was, summarized the applicable principles of law at para. 49 of her Reasons for Judgment. Those principles include the following: 1. The parent applying for a change in the custody or access order must meet the threshold requirement of demonstrating a material change in the circumstances affecting the child. 2. If the threshold is met, the judge on the application must embark on a fresh inquiry into what is in the best interests of the child, having regard to all the relevant circumstances relating to the child's needs and the ability of the respective parents to satisfy them. 3. This inquiry is based on the findings of the judge who made the previous order and evidence of the new circumstances. 4. The inquiry does not begin with a legal presumption in favour of the custodial parent, although the custodial parent's views are entitled to great respect. 5. Each case turns on its own unique circumstances. The only issue is the best interest of the child in the particular circumstances of the case.