By as early as 2028, it's projected to be Australia's biggest city. By 2050, it will have grown to 8 million — the size of London and New York. "It's the fourth-fastest growing developed city on the planet today," demographer Bernard Salt said. "If we don't invest, and continue with this rate of growth, then we collapse under our own weight. You end up with a Bangkok situation — where you have an extraordinary level of growth and congestion, and you simply cannot move around the city."