Up to four in five of the "congestion busting" infrastructure projects announced by the Morrison government are flowing to Liberal-held seats the government needs to hold at the election.

The Coalition is poised to top-up its $1 billion urban congestion fund in Tuesday's budget after embarking on a pre-election spending spree which has favoured traffic hotspots in Liberal marginal seats.

The fund, a key element of last year's budget, has just $23 million left in it after Prime Minister Scott Morrison spent the past six weeks making a string of announcements across capital city suburbs.

Deputy Prime Minister Michael McCormack, Foreign Minister Marise Payne, Prime Minister Scott Morrison and NSW Premier Gladys Berejiklian visit the Western Sydney International Airport. Credit:Louise Kennerley

A breakdown of the 45 projects shows 39 of them have been earmarked for Liberal-held seats or will be shared with Labor electorates. They include a series of improvements worth $261million in Melbourne with the key seats of La Trobe, which is held by Liberal Jason Wood on a 3.2 per cent margin, and Deakin - held by Michael Sukkar - getting six of the 14 projects announced for the city.