Hi Wonderer,
A lot depends on which part of Australia you live in and what affect the medication may have had on you. If it is the "family home" and your wife is listed as an owner, you cannot sell it alone, anyway.
In essence a verbal or written contract is binding if both parties knew what they were signing, its effect and that the contract did not remove or interfere with other rights or obligations under the law. For example, you cannot enforce a clause in a contract that says no stamp duty is payable on the purchase. Stamp duty must be paid by law and such a clause would not negate it.
I think an obvious and urgent first step for you is to see your local Legal Aid office. If you do not qualify (earn to much) then you need to pay for a conveyancer to help you.
There is obviously a lot at stake.
If you have merely changed your mind but knew what you were doing when you signed the contract, it is enforceable.
Good luck with it. John.
