A Melbourne Airport rail line is important, but the Victorian government says it needs to be properly planned after reports the commonwealth could go it alone on the project.

The Turnbull government is so keen on a rail link to Tullamarine, it could fund it without Victorian government assistance, the Herald Sun reports.

Acting Public Transport Minister Luke Donnellan says Victoria welcomes "the prime minister's sudden interest" in the state's infrastructure.

However, a joint state-federal study for the best route and funding arrangements needs to completed first, Mr Donnellan said.

"Airport rail link is an important project that we're currently making possible by building Australia's biggest rail project, the Metro Tunnel," he said in a statement.

"We would welcome funding for the project once planned, particularly since Victoria now receives only 10 per cent of infrastructure funding from the Turnbull Government."

The commonwealth is putting in $30 million and Victoria $10 million to look into the rail link.

Prime Minister Malcolm Turnbull on Tuesday told reporters a train to Tullamarine was "long overdue".