Just 15% of projects announced in the government’s multibillion dollar infrastructure budget splurge have been endorsed as priorities by Infrastructure Australia, with the Coalition instead funnelling billions into must-win marginal seats.

A Guardian Australia analysis of the 144 projects being funded by the government in Tuesday’s budget shows that just 21 are included on Infrastructure Australia’s current list of priority projects, accounting for $5.7bn of the approximate $16bn in new funding.

The IA list is maintained “to ensure that public funds are directed towards projects that will deliver the best outcomes for our growing communities”, with the list prepared after “rigorous and independent analysis”.

Almost $10bn in funding spread across 120 projects was not recommended by Infrastructure Australia with the bulk of this going to projects that span multiple electorates.

The analysis also shows that of the $6.4bn that is allocated to projects within a single electorate, more than half – $3.3bn – is directed to marginal seats.

This compares with fairly safe and safe seat groups, which received $1.2bn and $1.9bn respectively.

The largest single project is a $1bn commitment to faster rail between Sydney and Newcastle with the federal contribution to upgrade stations and rail lines at Wyong and Tuggerah, along with new bridges over the Wyong River. The faster rail project is one of those included on Infrastructure Australia’s priority list.

The spend is in the central coast seat of Dobell, which is among Labor’s most marginal and one that the Coalition is targeting at the forthcoming election. Dobell will also benefit from $336m in new funding for Pacific highway upgrades through Wyong town centre, and a separate $51m highway upgrade in Wamberal.

Urban infrastructure minister, Paul Fletcher, said the rail upgrade would contribute towards “faster, more frequent and more reliable services” between Sydney and Newcastle which he said was the busiest regional passenger rail corridor in Australia.

In the NSW marginal seat of Gilmore, which the government is hoping it can claw back from Labor with former NSW transport minister Andrew Constance running as its candidate, the Coalition will tip in an extra $352m for the Milton-Ulladulla bypass, bringing the federal government’s total commitment to the project to $752m.