@Captain_A : They will consider different facets of the relationship i.e. household arrangement, financial status, and social aspect. So I am trying to build the proof of relationship on each aspect.
In my case, I wanted to be on the safe side so I will provide official documents. On the financial aspect, we're planning to open a joint investment fund aside from the joint savings account.
I suggest you open a joint account, kahit maliit lang. Pero tama si @LokiJr, mas ok kung magpakasal na. Although, will they not scrutinize that the marriage happened because of the visa? =) haha.
Repost from @lock_code2004:
http://www.immi.gov.au/media/fact-sheets/35relationship.htm
FYI: They also consider de facto relationship between same sex. In theory, it should be easier to prove that you are in a genuine relationship if you two are of different sex.