Prime Minister Malcolm Turnbull will join Victorian Opposition Leader Matthew Guy on Tuesday to announce the policy, which would link the growing new suburbs south of Frankston to Melbourne’s broader train network.

Baxter is a stop on the spartan and unreliable Stony Point line, which runs diesel trains. Commuters have long called for an upgrade to modern electric trains on the line.

The announcement comes on the heels of Mr Guy’s promise last week to extend the Cranbourne train line to the fast-growing suburb of Clyde, as electoral battle for vital seats in Melbourne’s south-east heats up.

The Baxter line is also mostly in the federal seat of Dunkley, held by the Liberal's Chris Crewther but made harder to defend at the next election by this month’s boundary changes. The changes made the seat notionally Labor.

The plan would extend the Frankston railway line by eight kilometres as an electrified, twin-track railway to Baxter along the current Stony Point line.

There would be two new stations built, one at Frankston East, near Monash University's Peninsula campus and the Frankston Hospital, and one at Langwarrin.

Baxter station would also be upgraded, with the Coalition hoping to start work next year if it won November’s election.

The Turnbull Government committed $225 million towards the project in its 2018-2019 Budget and Mr Guy will pledge another $225 million for the project, which would also see the Moorooduc Highway level crossing removed.

Mr Guy will also promise on Tuesday to tackle other “major shortfalls” in public transport services in the area by improving bus services between Frankston East, Langwarrin, Baxter and Portsea and Hastings on the Mornington Peninsula.

“We need to ease the population squeeze, which is why I am so focused on planning for decentralising our population growth and that means more roads and more public transport,” Mr Guy said on Monday.

“If we are going to decentralise Victoria’s population, we need to provide more education and health services as well as build and upgrade rail lines to areas like Langwarrin and Baxter."

“A new station at Frankston East will improve access to Frankston Hospital for patients and families as well as health professionals, and it will also and help students get to the local campus of Monash University.”

Mr Turnbull said the promised spending was part of his government’s $17 billion infrastructure investment in the state.

“The approach would complement the Turnbull government’s $17 billion investment in Victorian infrastructure, which is possible because of a stronger economy," he said.

“Only a strong economy enables the government to guarantee the essential services that Victorians rely on."

“The business case for the upgrade, which was initiated by Mr Crewther and funded by the Turnbull Government, is currently underway and due for completion in 2019.”