It makes me feel very old when I hear of plans for an airport at Badgerys Creek in our state of New South Wales. Once again it is on the agenda. It is alternative for planes that now land at Sydney airport which is apparently overcrowded by too many flights. I have heard about an airport being built at Badgerys Creek for decades and nothing ever happens.
I don't know what an airport costs to build, but it would cost a lot of money. The money might be better spent on a train.
If high speed rail was built from Brisbane to Melbourne via Sydney and Canberra, there would be no need for a second Sydney airport. From Melbourne, the first leg could be to Melbourne Airport, and then there would be no need for expansion of the freeway to the airport. It could be incorporated into the never to be built Footscray to South Yarra underground train line.
Flying from airports is a miserable experience. Catching a train is a wonderful experience.