Melbourne, Sydney and Brisbane could all do with an airpark.
Melbourne needs one in the East or SE, Sydney in the North and Brisbane could do with one in the South or SW. Problem is of course distance and councils.
Everybody wants to be close to the city but the land is expensive and residents disapproving. Cheaper land further out is usually afflicted by transportation difficulties and lack of nearby amenities. Getting something approved as an airport takes the worlds most doggedly determined individual - councils seem to view the term as being akin to nuclear reactor and toxic waste dump.
But it can be done - either from scratch or at an existing airfield.
From Scratch: Buy some rural land as close as possible to railway lines and freeways etc and continue to run it as a farm etc but put up a couple of 'sheds'. The sheds house an aircraft or two or three. Orient landing strips away fro neighbors houses and don't do circuits there. After some time gradually formalize the nature of the property legally and eventually slip in the term airpark or even 'airstrip' or landing field. This is ideal for areas between Beaudesert and Brisbane, say Leppington SW of Sydney and fill-in areas such as the west or east of Cranbourne SE of Melbourne. This is a long process though. Slightly further away however this is already being done. Lethbridge west of Melbourne I believe already has expansion approval and it would be relatively easy to do the same on private land at say Lilydale or Coldstream as there is a big residential component nearby anyhow.
Existing Airfield: This has to be the way to go. Airports like Point Cook, Avalon lend themselves to such concepts. Even (at a stretch) the eastern side of Essendon Airport has a ton of vacant land off Lebanon St and Arvon Rd that would be perfect (ah, the serenity) if Mr Fox could be so persuaded. Other airfields like Parafield and Caboolture also have vacant perimeters that would be suitable, just takes a bit of persuasion and political savvy. This is far and away the easier option as the infrastructure is already in place and the airports are closer to transportation links and the amenity of the big cities.
And every time silly Kelvin Thompson opens his mouth complaining on behalf of neighbors about airport noise you can say that you are a neighbor and very much in favor of the noise!
Wouldn't mind having a crack at this myself!