I agree with paradise.
28 days makes it pretty stressful. If you have been living together for 12 months, and can prove the relationship has been continuing for at least that time period, you should be able to satisfy the requirements for the onshore 820 de-facto partner visa.
However, the prospective marriage visa 300, requires you to leave Australia, and apply outside Australia. Maybe not what you want to do. You can apply for the 820 inside Australia. You will be granted a bridging visa which will keep you legally in Australia until a decision is made.
Registered Migration Agent
Ausstart National Immigration