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. 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.

Infrastructure Australia’s chief executive Philip Davies saidthe modelling suggested that Melbourne would cease to be the world’s most liveable city unless it changed its approach to city planning.

“In the next 30 years, Melbourne will be comparable to the sizes of New York and London today,” Mr Davies said.

Of the report's 12 recommendations, the most critical is that governments increase their investment in public transport.

Mr Davies said Melbourne Metro, the $11 billion underground rail link through the CBD , is a good response to recent population growth after years of under-investment.

But the report argues that planning should start now for further enhancements, including another underground rail link through the city, signalling upgrades and tram route extensions.

“Now is the time to get on the front foot and plan for the growth we know is coming," he said.

The report suggeststhat some form of road user charging for trucks and cars must be introduced “within 10 years”, to better manage congestion and deliver revenue for better roads and public transport.

Premier Daniel Andrews has previously ruled out bringing in a user-pays road pricing system for Melbourne motorists.

One of its chief goals of the Andrews government's Melbourne blueprint for 2050 is the creation of a series of “20-minute neighbourhoods” , where people have the opportunity to work, shop and access schools and doctors within 20 minutes of home.

But Infrastructure Australia’s modelling found this plan is likely to fail in all three future scenarios, because Melbourne will simply be too big, and commuting distances too great, for most people.

Infrastructure Australia's three scenarios for Melbourne in 2046

The LA model:

More than one million extra people – or 40 per cent of projected population growth to 2046 – will live on the city’s edge in 2046, under a planning scenario that sees unfettered low-density development.

Melburnians will rely more heavily on cars to get to work, with only 3 per cent of jobs accessible within 30 minutes by trains, trams or buses.

They will be forced to travel further distances; hospitals, schools and universities will be less accessible in 2046 than they are today.

Melbourne’s fast-growing west will bear the brunt of the growth, with population set to rise by a whopping 96 per cent, fuelling 69 per cent job growth.

If Melbourne followed a Los Angeles-type sprawl, would traffic jams like this become the norm? Twitter/@ABC

The New York City model:

A compact, higher-density vision for Melbourne will concentrate jobs and housing within 15 kilometres of the city centre, and will drive up public transport use.

An extra 2.2 million people will be located in established areas, with new housing built at medium to high density along public transport routes.

This is the only planning model for Melbourne in which access to hospitals, schools and universities across would improve by 2046.

Potential equity problems could occur under this model, especially if jobs are concentrated in inner Melbourne at the expense of outer suburbs that generally have cheaper housing.

New York City offers more compact and high-density living. Ross Duncan

The London model:

A medium-density model that spreads the population growth more evenly and puts jobs closer to where people live.

Seventy per cent of growth will occur in established suburbs, almost two million people.

Melbourne’s population centre will move west, as it absorbs an extra 900,000 residents. Jobs will also grow there, at employment hubs in Werribee, Sunshine and the Tottenham industrial estate.

New housing will be built near rail hubs such as Sunshine and Footscray, or at repurposed industrial sites like Essendon Airport.