Prime Minister Helen Clark is heartened by the latest opinion poll which she says shows Labour retaining its core support.

TV One's survey, released last night, showed National down one point to 51 percent support and Labour up two points to 37 percent.

Miss Clark today said the poll continued a trend of Labour climbing back up in support.

"It puts us within around a point and a half of our 1999 election result and that shows a party very strongly keeping its base."

Miss Clark believed National's support would drop.

"I don't believe 51 percent can be held at all. I think you get into a campaign environment. Things close up," she said on TVNZ's Breakfast programme.

"So I think it's game on. I'm looking forward to the time when we down tools in Parliament and get into full-blooded campaign."

An election must be held by November 15.

Labour's gain in the poll seems to have come at the expense of its ally the Green Party, which went down from 6 percent a month ago to 3.5 percent.

At that level, the Greens would not have any seats in Parliament because they do not hold an electorate and have to pass the 5 percent threshold under MMP.

But Miss Clark said she expected the Greens' support to swell in the campaign.

"The small parties get more publicity during a campaign as well. I would be extremely surprised if the Greens polled under 5 percent."

New Zealand First also suffered with just 2.6 percent, meaning it would disappear as well unless its leader Winston Peters can win Tauranga.

The other small parties barely registered, and if the results were translated into seats National would have a majority with 66 against Labour's 48.

The others would be the Maori Party with four seats and United Future, ACT and the Progressive Party with one each.

In the preferred prime minister stakes National's leader John Key maintained his lead over Helen Clark, although Miss Clark was up from 31 percent a month ago to 33 percent and Mr Key dropped two points to 36 percent.

The poll was better news for the Government than Saturday's Fairfax Media survey, which put National on 54 percent and Labour on 35 percent.

Both polls were carried out after National's problems with secretly taped conversations at its annual conference, and showed Labour claims of a secret agenda were not having much of an impact.

The TV One poll questioned 1000 voters and had a margin of error of plus or minus 3.1 percent.