By 2050 Sydney will be a city of 9.08 million and Melbourne 9.61 million.
Both cities though have enough room for future growth even without sprawl.
Tokyo, with a population of 9 million, is spread across about 2,000 sq km. London, with a similar population, occupies just 1,500 sq km.
Sydney by contrast has sprawled across 12,000 sq km and Melbourne 10,000 sq km.