What are the laws regarding de facto relationships between flat mates?

I'm curious because I own my own 2 bed unit and am looking at getting another person in the other room. Now dad tells me that after 6 months living with someone they have claim to your assets due to de facto laws. Is this true and can I put a clause in the tenancy agreement stating that the relationship is (agreed by both parties of course) not to be a de facto relationship unless stated and witnessed by a third party into the future?