I used to work in finance for a property development company so I have a tiny bit of exposure to it - my understanding is that it is pretty much what you said - you may be able to obtain a town planning scheme for an area for a period of time but these can always be changed down the track. With the urban sprawl that Australia is experiencing it's not inconceivable that areas that are now rural or semi-rural will become urban in 20-30 years time. Part of what my company did was buy up bits of land in "woop woop" areas and turn them into suburban subdivisions, and while alot needs to happen to do this (ie governments need to agree to build roads, schools, other infrastructure etc) it does happen quite a lot.

Sorry, I know my response is a bit vague and probably not telling you much. I would try and speak to a planner in the council and see what they can tell you..