Hi Mal,
Seek professional advice from a lawyer, not on here!
Here are 2 sites that may get you started (NSW law), eg a bit of light reading:
http://www.austlii.edu.au/au/legis/nsw/consol_act/rpa1900178/
http://www.legalanswers.sl.nsw.gov.au/guides/tenants_rights_manual/other_tenants/squatters.html
