Prime Minister Malcolm Turnbull's harbourside electorate of Wentworth has the biggest proportion of earners who will benefit fully from income tax cuts unveiled in Tuesday's budget.

More than a third of income earners in the Prime Minister's seat will get the $315 annual relief from the government's decision to lift the threshold at which the second top tax rate applies from $80,000 to $87,000 - the largest share of the 150 electorates in federal parliament.

The Australia Institute has used income data from the last census indexed to 2015 to estimate what proportion of earners in electorates would benefit from the income tax cuts.

People in Melbourne Ports, held by Labor's Michael Danby, are the biggest Victorian winners with 28 per cent of earners set for relief.

The seat includes Albert Park, St Kilda, Caulfield and South Melbourne.

The next three winners are the eastern suburban blue ribbon Liberal seats of Higgins (27 per cent of earners over $87,000), Kooyong (26 per cent) and Goldstein (24 per cent).

The fifth-biggest winner, with 21 per cent set for the tax relief, is the seat of Melbourne, held by the Greens' Adam Bandt.

At the other end of the scale just 6 per cent of voters in the rural seats of Mallee and Murray will enjoy the $315 saving.

"This tax cut is highly selective, with very few receiving the $6 a week tax cut – all at the top end of the income spectrum," Senior Economist at The Australia Institute, Matt Grudnoff said.

Mr Grudnoff said Victoria was different to the rest of the nation because the wealth was distributed among three different parties. Across Australia Liberal-held seats are the top beneficiaries, Labor seats in the middle and Nationals held seats at the bottom.

The electorate of Fowler, only 35 kilometres across town in western Sydney from Mr Turnbull's seat has the nation's lowest proportion of taxpayers who qualify for the tax cut.

In that seat, which takes in the suburbs of Liverpool, Cabramatta and Green Valley, just one in 20 earners get more than the new $87,000 threshold.

The uneven distribution of budget tax relief is underscored by the contrast between the seats held by the prime minister and his deputy, Barnaby Joyce. While 34 per cent of earners in the electorate of Wentworth qualify for the full income tax cut, only 8 per cent of earners in Mr Joyce's regional NSW seat of New England have an income above $87,000.

The Treasurer, Scott Morrison, said the aim of the tax change was to prevent "average full-time wage earners" from graduating into the second top tax bracket of 37 per cent.

"The government is not claiming that this is a big cash splash, it's not … [but] if you have a middle income tax bracket, well middle income earners have to be in it," he told the Tax Institute during his post-budget hustings this week.

The income tax relief in this year's budget will cost nearly $4 billion over four years.

Those on very high incomes will get an added benefit when the temporary budget repair levy of 2 per cent on taxable income over $180,000 is withdrawn, as planned, in the middle of next year.

Separate analysis data from job search engine Adzuna showed Canberra, Sydney and Brisbane had the most advertised vacancies among the state capitals, with salaries over the new $87,000 tax threshold.

Across industries, the IT sector had the most vacancies with salaries over $87,000 (70 per cent) followed by engineering (60 per cent).

The sectors with the lowest proportion of vacancies with salaries over $87,000 was customer service (1 per cent), maintenance (1 per cent) and retail (4 per cent).