Melbourne is on course to lose its status as one of the world’s most liveable cities, unless smart decisions are made now about how to accommodate almost three million extra people, Infrastructure Australia warns.

The federal government’s infrastructure adviser says in a new report that Melbourne and Sydney are at watershed moments in their histories, as population growth pushes them to a future where they will resemble megacities more like today’s Hong Kong, London and New York.

Melbourne CBD is fast becoming dominated by high rises. Credit:Paul Rovere

On current projections, Melbourne’s population will grow by 2.8 million to 7.3 million by 2046.

Infrastructure Australia’s report, Future Cities, released on Friday, models three potential futures: a centralised high-rise version, where growth is concentrated in the inner city and along major transport corridors; a sprawling version based on continued low-density growth in the outer suburbs; and a medium-density scenario that involves more intensive development in Melbourne’s middle suburbs.