In every instance prior to constructing a (legal) dwelling in Australia, one must first ensure that there are no statutory planning restrictions in place. You mention in your situation that there is a
BMO (Bushfire Management Overlay) affecting the site, but fail to mention the zoning or size of the land. The latter may prove to be prohibitive, too. Of course, it may be that the site you have chosen is free from any planning restriction at all, and that you are indeed able to (legally) commence construction. Either way, if you need a hand interpreting
your local government Planning Scheme, please feel free to contact me for some free advice.
One very cheap dwelling option (relatively speaking), is to purchase an older-style school portable building, shift it to your site, and retrofit it to meet
Class 1a objectives and standards.
These types of structures can often be bought for around $5,000. Some people buy two or three of them, 'join' them together in a modular-type fashion, and end up with a very comfortable dwelling.