There is renewed focus over the project's cost, after a Parliamentary Budget Office assessment earlier this month found the first two stages of the orbital train line around Melbourne will not provide adequate value for money.

The two main sections, dubbed SRL East and SRL North, will form a 60km metro line through the city's middle suburbs across 13 stations.

The independent analysis requested by the state opposition found construction costs would be about $96 billion, with every $1 spent returning social benefits of between 60 and 70 cents.

However, it said there could be a net positive value up to $17.3 billion if the project includes "all expected benefits" such as infrastructure savings, better social inclusion and access to employment.

So far the first stage of the project is expected to cost $34.5 billion and has been funded to the tune of $14 billion.

The state government is contributing $11.8 billion and the federal government has committed $2.2 billion.

Premier Jacinta Allan said discussions were continuing with the commonwealth over funding and Victoria cannot afford not to build extra transport connections.

She said the project "absolutely stacks up" after winning support at two state and one federal elections.

"Melbourne is the biggest city in the country, the biggest city in the country, and it deserves to get its fair share of infrastructure funding from its national government," Ms Allan told reporters in Burwood on Monday.

"They're the conversations I've been having and will continue to have with the federal government."

Opposition spokeswoman Jess Wilson claimed the money for the project "doesn't currently exist".

"The premier is focused on talking about a suburban rail loop that nobody seems to want, it's unfunded," Ms Wilson said.

"We are constantly seeing from this government talks of the federal government stepping in and providing significant amount of money for this project."

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