National finance spokesman John Key is slowly creeping up the preferred Prime Minister stakes in the latest Herald DigiPoll survey and has pushed New Zealand First leader Winston Peters from third place.

Mr Key is now preferred by 6.3 per cent, up from 4.3 per cent, and just ahead of Mr Peters on 6.1 per cent. Although he is well behind his leader, Don Brash, on 22 per cent, the fact that he is third ranked - and rising - may revive hopes of those who want him to unseat Dr Brash before the next election.

The poll also indicates that while Prime Minister Helen Clark is streets ahead of anybody as preferred Prime Minister, very little separates Labour and National in voter popularity.

And it shows that the importance of the tax issue to voters has dived since it held the position of prime concern during the election campaign last year.

Labour has 43.4 per cent support, 1.2 points ahead of National.

Despite a difficult start to the year for Labour - trouble around election spending, MPs David Benson-Pope and David Parker, and the economic slowdown - the relative positions have not changed much since the last Herald-DigiPoll survey conducted in November and December when 2.1 points separated them and Helen Clark was preferred by 58 per cent of voters, now 57 per cent.

Marginally fewer people think the Government is heading in the right direction compared to those who don't: 42 per cent believe it is heading in the right direction; 43.8 per cent think it is not; and 14.2 per cent dont' know or refused to say.

Before the September 17 election, Mr Key barely rated a mention in leadership polling. The only other National MP who registered in the polling was former leader Bill English but, even then, usually in the fractions.

Mr Key began rating after the tax-focused election campaign which thrust him centre-stage. Mr English, who has led National in the House this year in terms of command performances, is rated by only 1.2 per cent.

Dr Brash put a lid this year on speculation about leadership alternatives when Mr Key was perceived by the caucus to have been stimulating such talk.

Maori Party leader Tariana Turia raised the leadership issue on Saturday on TV One's Agenda show, endorsing Mr English as a great leader for National.

There have been minimal movements for other parties in the latest poll: Greens 5.2 per cent (down 0.1 point); New Zealand First 3.9 (down 0.1); Maori Party 2.1 (up 0.6); United Future 1.5 (up 0.2) and Act 1.1 (down 0.4).

Health has returned to being the issue of most importance, a position it held before tax overtook it during the campaign. The tax issues rates fourth most important after health, law and order and the economy.

During the election campaign, Labour countered National's tax cuts with an expansion of the working for families set of policies - dubbed tax relief - which began to take effect on April 1.

A gender breakdown of the polling sees Labour keep its high level of support among women (48.2 per cent of women support the party); and growing support for National among men (48.1 per cent compared with 43 per cent in the last poll before the election).

* The percentages are of decided voters. In the party vote question, 16.9 per cent of total respondents either did not know (8.4), refused to answer, or said none. In the preferred prime minister question, 22.1 per cent of total respondents either did not know (9.4), refused to answer or said none.