Sunfish,
Sydney and Melbourne could keep their current runways, and would not require any new ones if there was a fast train service between the two cities. If the train could do it in three hours, then number of flights would reduce dramatically. see
http://www.traveller.com.au/worlds-busiest-airline-flight-routes-melbournesydney-now-worlds-second-busiest-h0e7ha
Think outside the square.