I'm just a forum user who went through this partner migration process myself, but this is how I see your situation:
you've been a couple since December 2011 and you moved in together just recently (end of February or March 2013). So you've been together a bit more then a year, and now the question is whether you were only dating since December 2011 to February/ March 2013, or did your relationship turn into a de facto relationship at some point earlier than in February/ March 2013. If it did, you need evidence to back up your claims, some sort of the proof. Remember, cause you're not married, you need evidence for all the months you claim you've been in a de facto relationship and in all four categories.
To be eligible for a partner visa on the basis of a de facto relationship at the time you apply, you and your partner must have been in the relationship for at least the entire 12 months before the date you lodged your partner visa application. Dating unfortunately doesn't count towards the 12-month relationship requirement.
The 12-month relationship requirement at time of application lodgement does not apply if at the time of application for the visa your de facto relationship was registered. You can register your relationship in NSW...
Now it all depends on the evidence you have since your relationship started and on the date your relationship turned into a de facto (when you moved in together or earlier?). If you register your relationship, you don't have to prove you have been living together for 12 months but still you need to back up your claims and prove with evidence your relationship is genuine, continuing, exclusive...
Her visa expires at the end of August, so you'd have five months of cohabitation and good evidence, plus you'd register your relationship, plus you'd find some evidence to back up your claims you've been in a relationship since December 2011.
There are some awesome threads on evidence on this forum, check them out. Be creative with your evidence, you wouldn't believe what you can find in your old bank statements etc...
Other useful evidence: being nominated drivers on car insurance policies, both being superannuation beneficiaries, wills, etc.
Once you see how much evidence you have, it would be good if you discuss your case with a good registered migration agent. Sometimes it's worth to pay a bit more per advice and get it from someone with an excellent reputation and experience. I've been burned before, got an advice that wasn't correct, so I don't trust the agents blindly just because they're registered.
And then you make a decision on what will you do, apply for a de facto visa or a PMV.
But definitely register your relationship straight away, here are the details:
NSW Registry of Births Deaths & Marriages