I just recently enquired with Gympie regional council about adding a granny flat onto the back of a large block. The planning officer was really vague, she basically said the block only needed to be greater than 800sq mtrs. She then said I'd have to apply for approval for the building and for the block to become multi-dwelling. When I questioned her about all this, she said it's case by case. So I'd have to buy the property, then apply and be potentially rejected. I'd need to know that i could do it before buying the property, so this seems impossible.
Gympie treats it like most other Qld Councils. Very hard to get through unless you have the correct zoning and then those blocks are probably better used for townhouses or units etc.