That's why the Family Law Act requires you to receive independent legal advice before your prenuptial agreement becomes binding - to ensure that its fair and that you understand what you're getting into. Also have a read of:
I just noticed that you are located in WA.
Similar rules apply in WA however the governing law for de facto couples is the Family Court Act 1997. I have updated the links to my answers with the relevant sections of the Family Court Act 1997.
Firstly, there are certain legal requirements that must be satisfied for the agreement to be binding. Click here to see what these requirements are.
Secondly, in certain circumstances the courts have the discretion to set the agreement aside. Click here to see when the courts can set agreements aside.