I'm assuming you're in Vic. My mistake: they don't use the legal term de facto. The definition that they use for spouse/partner is:
http://www.sro.vic.gov.au/sro/srowebsite.nsf/rebates_fhog_FAQ.htm#6
What do you mean by spouse/partner
A person is a spouse of another if they are legally married to each other. A person is a partner of another if they are in a domestic relationship regardless of gender.
Get advice from your lawyer. Living together will definitely be considered a domestic relationship.
Alex