Prenups orBinding Financial Agreements are legally binding and must be followed by the courts. But there are circumstances where they won't work - they may be defective, assets not disclosed, interests in trusts not discclosed, wording not meeting the legislation, independant legal advice not obtained, lawyer not explaining certain aspects etc.
see s 90k
http://www.austlii.edu.au/au/legis/cth/consol_act/fla1975114/s90k.html
+1
prenups are an american thing,
binding financial agreements (is this the full terminology) are not worth the paper they are written on if cricumstances change, eg kids.
Just protect yourself as best you can from day one OR be prepared to lose it OR find someone richer!!!