Dear NJ,
The law that governs binding financial agreements if the Family Law Act 1975 or if you are in WA and in a de facto relationship, the Family Court Act 1997 (WA). Any testamentary declarations or clauses in a BFA are not enforceable under this legislation.

The appropriate way to achieve what you are requiring is to have both your wills re done to include a “Right of Residence” along the lines of the following:

Right of Residence
In the event of my death, XXXX shall have the right to reside in the property located at [property address] (the ‘Property’) currently held by me and XXXX as tenants in common, or such other property purchased by me and XXXX in substitution, until such time as XXXX chooses to leave the property, or in the event that they remarry, or in the event that they enter into a de facto relationship, or XXXX reaches the age of 70, such right to reside being conditional on XXXX paying all council rates, water rates, insurance premiums, strata levies if applicable and reasonable maintenance costs.
Kind regards,