Hi Cate, I don’t know what the laws pertaining to this would be in WA. We had a similar problem in a strata I used to live in (NSW). This is how we approached it:
Give the present owner a decision of the EC that there are unauthorised works that must be undone:
During the sale process, a prospective new owner will require a notice from the OC re the unit. In NSW this is under s108 of the relevant act. Ensure that any prospective buyer is notified of the unauthorised works and that the works will have to be undone (even if the new buyer must pay for them).
The new buyer can then negotiate with the present owner. The important point is to get to prospective new owner as soon as possible.