if you buy .net.au domains , go for crazy domains. $8 per year. how ever they do not have instant activation for any domain...
My all domains are with ventralp,, bcz they are the cheapest, when it comes to .com.au,,,,,
i'm not going to recommend anyone, nor who i use because i just want to put forward a point.
don't choose on price, do it on them having an admin area that you find easy, managable, because if you get down the track and you have 100+ domains and then want to change it can be a long process to do that and move them.
its worth an extra $1 or so to just have something that you find suits your way of organising things.
and the domain name registration price CAN be negotiated ! if you are going to do alot.