Malcolm Turnbull and Bill Shorten have fired the gun on a pre-election spending war on transport as they clamour for voters’ support ahead of the next federal election.

As Mr Shorten unveiled a third multimillion-dollar transport promise in Western Australia on Thursday, the government upped the ante by revealing the May budget’s big-ticket infrastructure item – a $5 billion pledge to help build a long-discussed railway line to Melbourne Airport.

The promise, which would require massive joint funding from the Victorian government and possibly the private sector, came days after Mr Turnbull committed to a billion-dollar upgrade of the M1 Motorway along parts of southern Brisbane and the Gold Coast.

First raised as an idea in the 1970s, the rail link to Melbourne Airport would require a $5 billion pledge from the Victorian Labor government, which was lukewarm in response to the announcement.

“It’s good the Prime Minister has finally found Victoria on a map, and we’ll accept this funding given how much he short-changes our state,” Victorian transport minister Jacinta Allan said.

Mr Turnbull said he hoped construction could start after 2020.

“The important thing is to get on with it. $5 billion is the type of money and the type of leadership that we need to get this project started,” he told 3AW.

It is believed the project could cost as much as $15 billion in total, in which case private investment would also be required.

Shorten on WA blitz

While the government has grappled with the 30th consecutive Newspoll loss and internal fights over energy and immigration, Mr Shorten has blitzed Western Australia this week, visiting 11 electorates and making local road project promises aimed at dislodging two of the government’s key frontbenchers at the next election.

Labor committed $108 million to fund an expansion of Perth’s Mitchell Freeway, one of the city’s busiest roads in Attorney-General Christian Porter’s marginal seat of Pearce.

The opposition also pledged $64 million to extend another major road in Liberal frontbencher Michael Keenan’s seat of Stirling, which he holds by a mere two per cent.

On Thursday, Mr Shorten said Labor would also spend $240.5 million to extend the Armadale rail line, in Perth’s south-eastern suburbs.

“The reason why we’re making our announcements well before any election is because I’ve got respect for the voters,” Mr Shorten said.

“I think voters hate it when you just turn up at the last minute and make big announcements which you know, they’re appropriately cynical about.”

The cash splash followed further commitments Labor made last year as part of a $1.6 billion infrastructure fund designed to compensate WA over its share of the GST.

Election preamble

While the Prime Minister maintains the federal election will be held in the first half of next year, Mr Turnbull has reportedly instructed the Liberal Party to finalise all pre-selections by June at the latest.

Collectively, the major parties pledged $6.5 billion in transport spending in four days.

On Monday, Mr Turnbull was in Queensland to spruik the $1 billion promise to upgrade the M1.

The project will benefit voters in the electorate of Forde, in Brisbane’s south, which is held by Liberal MP Bert Van Manen on a knife’s edge margin of 3.75 per cent.