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

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.