And the government has defended its decision not to build a second railway station in South Yarra as part of its massive $11 billion rail tunnel under inner Melbourne, saying the huge extra cost cannot be justified.

At the second day of hearings into the environmental effects of the project, barrister Stuart Morris, QC - who argued in 2014 for construction of the East West Link road tunnel - has laid out the government's justification for the twin train tunnels under Melbourne's CBD.

Acting for Public Transport Victoria, Mr Morris said Melbourne's population was growing by 2.2 per cent a year - faster than the rest of the country, which had seen a 1.8 per cent growth recently.

"Melbourne is growing very quickly by world standards," said Mr Morris, the former president of the state planning tribunal. He pointed out that, when much of Melbourne's rail network was laid out in the 1880s, the city's population was just 280,000, compared to its current population of 4.5 million.

He said the tunnel - which would run from South Kensington to South Yarra, beneath some of the city's most jobs-rich suburbs - would allow better access to a city that would double in size by the year 2051.

He said the population increase was largely due to overseas and interstate migration to Melbourne. The growth meant that "this project is not just a good idea, rather it is absolutely essential".

The project was first proposed, in an earlier form, during Steve Bracks' time as premier, and later by John Brumby. But it was never funded by Labor and was ditched by the Baillieu government.

It was resurrected by Daniel Andrews when Labor returned to office in 2014 and is now expected to commence construction in 2018.

PTV argues the project will provide massive relief to rail lines in the city's outer west in particular, where train use has seen the biggest jump in the past decade, by providing more space on the rail network and allowing Metro Trains to run more services.

The Werribee, Williamstown and Sunbury lines have increased in use by more than 100 per cent over the past decade.

"Pretty much the last 10 years has been a massive increase in metro train [use]," Mr Morris said, due to the road system "reaching its limit" and commuters looking for alternatives.

While the project will bring benefits for Melbourne, there will also be losers.

The government admits that some commuters wanting to interchange at South Yarra may be disadvantaged by the lack of a new station as part of the project. The proposed new rail line will pass under South Yarra, metres from the existing railway station, but will not stop there.

The state opposition, federal Financial Services Minister Kelly O'Dwyer and Stonnington Council have all argued it would be a bad outcome if a new interchange is not built at South Yarra.

But Mr Morris said there were many who would be disadvantaged by the construction of a second South Yarra railway station.

Those not wanting to stop at South Yarra would have a minute or more added to their journeys as a result of stopping, he said.

And the number of minutes lost to travellers who did not want to stop at South Yarra exceeded the numbers of minutes gained by those wanting a new station, he said.

The government argues a new railway station at South Yarra as part of the project will cost at least $700 million and up to $900 million. The South Yarra area is already one of the city's best served locations for trains and trams.

"Clearly there are some benefits [to having an interchange]," Mr Morris said on behalf of PTV. But the net benefits of building the new railway station would not be sufficient to justify it, he said.

Hearings into the impact of the proposed rail tunnel will continue for the next seven weeks.