First thing you NEED to get seperate bank accounts as in there eyes thats a sign you are together.

Second contact them and ask to be assessed for Section 4(3) of the social security act 1991 'Seperated under one roof'.

You will need to show them that you meet the five factors for determining seperation under one roof.
The factors are
1/ Financial aspects of the relationship - eg you need to show he is not supporting you like he would have in a relationship
2/ Nature of the household - eg show the effort you have put in to separate yourselves within the household.
3/ Social aspects - New relationships ect ect
4/ The presence or absence of a physical relation - eg if you are sleeping together it would appear you are in a marriage like relationship
5/ Nature of the commitment - eg how have you distanced yourselves form one another physically and emotionally.

Hope this helps also its good to get a Advocate to write up a formal letter pointing out these to centrelink to goodluck hun