Is unsubstainable because there just isn't the need for four carriers. In the UK we have nearly three times the population, probably 500 million people within two hours in our catchment area but only two LCCs. The aviation infra structure needs looking at as well. Having worked at Tulla it's a disgrace. I used to have to walk from Tulla to Airport West to get a tram. I'm in Manchester now you can get a bus, train tram you name it. Sydney is no better with rip-off train prices. If you make it easy to travel people will. As much as I love Australia, aviation wise they don't half shoot themselves in the foot.