Section 56(4) of the Family Law Act sets out the factors for setting aside the terms of a separation agreement or any other domestic contract. A court will be inclined to set aside an agreement:
(a) if a party failed to disclose to the other significant assets, or significant debts or other liabilities, existing when the domestic contract was made;
(b) if a party did not understand the nature or consequences of the domestic contract; or
(c) if it is not in accordance with the law of contract.
http://www.canlii.org/en/on/laws/stat/rso-1990-c-f3/latest/rso-1990-c-f3.html#sec56subsec4_smooth