National is so far ahead of Labour in the Herald's post-Budget poll that it would win in a landslide if an election were held tomorrow.

The Herald-DigiPoll survey shows 50.9 per cent of decided voters supporting National.

Translated into votes, this would enable National to govern on its own, with 64 seats out of 122.

Labour is more than 17 points behind National, on 33.6 per cent.

Labour has been trailing National in most major polls, but this poll taken during the week after the Budget widens the gulf between them, putting National up about 7 points and Labour down about 7.

National leader John Key is ahead of Prime Minister Helen Clark as preferred Prime Minister. The poll shows 45.5 per cent support for him (up 9.3 points) and 42.1 per cent for her (down 5.6).

This reverses Helen Clark's lead in the previous Herald poll, in February, and reinforces the trend that emerged two weeks ago when TV3's poll showed her dethroned as preferred PM for the first time in eight years in a major poll.

The Government did not have an easy run into the May 17 Budget. The Australian Budget contained tax cuts for the fifth consecutive year, reigniting the issue in New Zealand.

Labour had hoped the "free money" on offer in the expanded KiwiSaver scheme would reverse its fortunes.

The poll appears to have dashed this hope, but as KiwiSaver does not start until July and some of its benefits will not take effect until next year - election year - it may still produce some electoral advantages for Labour.

Labour has also had to contend with unhappiness over interest rate rises, unhappiness among exporters with the rising dollar and law and order matters, including gangs.

There is also a hint that the anti-smacking bill may still be having an effect - the vocal Destiny Party registered 1.5 per cent support in the poll, more than most of Parliament's minor parties.

But unless a party wins a seat, it has to get more than 5 per cent of the party vote before getting any MPs in Parliament.

New Zealand First leader Winston Peters had 5.4 per cent support as Prime Minister, suggesting he could drag his party over the 5 per cent threshold even if he doesn't stand in an electorate.

The poll shows a gender factor for National - it has 54.4 per cent support from men questioned in the poll. Labour has 29.3 per cent of male support. Among women, the split is more even - 44.5 per cent for National and 40.7 per cent for Labour.