Opposition parties say they will not be dwelling on the result of a new poll that shows Prime Minister John Key's defeat in the flag referendum has not dented his party's support.

The Labour Party has fallen below the crucial 30 per cent threshold for the first time since the election in the latest TVNZ Colmar-Brunton poll.

It showed that National had risen three points to 50 per cent - its highest result in the poll in a year.

If an election was held tomorrow, National would be able to govern alone.


Labour fell 4 points to 28 per cent. The drop below the 30 per cent threshold will be a significant setback for Labour, especially because it has made major policy announcements this year.

This morning Green Party co-leader James Shaw told the Herald that the poll was not great news for Labour, but there was no need for panic on the left.

"You do have to take them [polls] seriously, but you also have to look at the aggregate of all polls and over a period of time so that you can actually see what the trend is ... yes, take it seriously but don't sweat any one poll too much."

New Zealand First leader Winston Peters rubbished the accuracy of the poll, and said that despite the high support, National were very beatable next year and would be worried.

Labour unveiled a multi-billion dollar plan to give every New Zealander three years of free tertiary education in January. It also revealed some of its high-level Future of Work Commission findings.

Labour leader Andrew Little said the result was "obviously disappointing" and reflected a "bad couple of weeks" in March.

"I think it's a question of us knuckling down, understanding the need for a clarity of message, and sticking to the things that are important to New Zealanders," he told the Herald.

In late March, Mr Little was mocked in Parliament after appearing to lay the blame for a migrant boom on ethnic chefs. Labour's consideration of a universal basic income in New Zealand was also questioned because of its potential cost.

Mr Little and most of his caucus opposed a change of flag in the referendum last month, and the party made a strong stand against New Zealand signing the Trans-Pacific Partnership (TPP) -- a stance which caused some pro-free trade MPs to break ranks.

That meant Mr Little (7 per cent) was overtaken in the Prime Minister poll by New Zealand First leader Winston Peters, who moved up 1 point to 10 per cent.

National's positive result came despite Mr Key's public advocacy for a change to a silver fern flag, which was soundly defeated in a referendum last month.

Mr Key's preferred Prime Minister ranking was barely moved, falling one point to 39 per cent.

This year, the Government has been promoting the TPP deal around the country in a series of roadshows.

The April 6 cut-off date for the poll meant it was unlikely to have taken into account any possible fallout from the Panama Papers scandal, which shone a light on New Zealand's tax-free foreign trusts.

Despite leading a strong campaign against a flag change, New Zealand First's support was nearly unmoved, falling 1 point to 9 per cent.

The Green Party, which announced a policy to make Kiwibank more competitive, rose 2 points to 10 per cent.