Family & de facto law, financial agreements, consent orders
So which one of them is actually LIVING in the place?..
In the eyes of the law, they have separated... Whichever one is living there, that person has the right to live there without the other coming around uninvited.. If changing the locks is what is necessary to achieve that, then that is okay..
Paying half the mortgage or being a co owner doe's not give the other person the right to enter that property
That doe's not remove the persons who left financial interest in the place though, & that can be addressed either through agreement or court orders..
The person that left should seek legal advice about continuing to pay half the mortgage though..