Prime Minister John Key believes voters have concerns about Act leader Don Brash holding the balance of power because he is an extremist, and they see the Maori Party as positive because it has worked constructively within the system.

He told the Herald that if he formed another government after the next election he would like the choices he has now - two major support parties to call on, the Maori Party or Act. "That has been useful and good for New Zealand." He also has the backing of United Future's sole MP, Peter Dunne.

Mr Key said he did not want to see Act dominate the Government.

The PM was commenting on the Herald-DigiPoll survey which showed 11 per cent of voters believed a Don Brash-led Act Party in coalition with National would leave New Zealand better off, compared with 47.3 per cent who believed it would be worse off. Thirty per cent felt it would make no difference.

Another question indicated a positive view of the Maori Party by general voters, with more than 60 per cent saying it was a positive force for Maori.

Mr Key said the reason nearly half thought a coalition with Dr Brash would leave NZ worse off was that they thought he could hold the "whip hand" and "because he is extreme".

"I believe National can work constructively with Act but Act can't dominate the Government."

He said the general view of the Maori Party was that it was taking a constructive role to try to address the negative social statistics for Maori in health and education.

"In the end, I think that New Zealanders recognise that to make real change you need to be inside the tent - and throwing stones from outside may create noise but it creates very little change."

Mr Key said National had yet to have a discussion since the change of leadership in Act about how it was going to handle the Epsom electorate.

In April, while Epsom MP Rodney Hide was still Act's leader, the PM said National would again campaign primarily for the party vote, as it did in 2008 - a clear message to National supporters that they could best help National by giving their electorate vote to Act.

Mr Hide has held the seat for two terms, allowing the party to claim the percentage of representation it received in the party vote across the country, even if it falls under the usual 5 per cent threshold, as it did in 2008.

With 3.65 per cent of the vote, and the Epsom electorate, it has five MPs in the 122-seat Parliament.

Since April, Dr Brash has ousted Mr Hide and on Saturday his preferred candidate, former Auckland City Mayor John Banks, was endorsed as Act's Epsom candidate.

On the back of the coup, Act's ratings improved 0.9 per cent to 1.7 per cent in last week's Herald-DigiPoll survey.

Dr Brash said he was surprised by the result.

He said the results showing 47 per cent believed a National coalition with him would leave the country worse off showed "we still have some work to do".