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

But the electorate of Fowler - 35 kilometres across town in western Sydney - has the nation's lowest proportion of income earners 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, modelling by The Australia Institute shows.

The uneven distribution of budget income tax relief is also highlighted 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 are over the new threshold.

The Australia Institute 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.

The electorates of North Sydney, Warringah and Bradfield on Sydney's north shore, along with the inner-city seat of Sydney are among the 10 Australian electorates with the highest proportion of income earners over the $87,000 threshold. The seats of Curtin (WA), Durak (WA), Melbourne Ports (Vic) and Higgins (Vic) along with the seat of Canberra were also in the top 10.

At the other end of the income scale, four NSW seats ranked in the 10 electorates with the lowest proportion earning more than $87,000. They were Cowper and Page on the state's north coast along with Blaxland and Fowler in western Sydney.

"This tax cut is highly selective," said Matt Grudnoff, a senior economist at The Australia Institute.

"On the eve of an election it is striking that such a small proportion of voters in most seats benefit from it."

Across Greater Sydney, about 17 per cent of income earners will get the full benefit from the budget's income tax cut.

The Treasurer, Scott Morrison, said the aim of the tax threshold 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).