Key Points:

National has widened its lead in this month's Herald-DigiPoll survey, recording its highest support in a year, while Labour has dropped to just above 30 per cent with the election no more than 3 1/2 months away.

The poll is the first to be completed since New Zealand First leader Winston Peters became embroiled in fund-raising controversies, but his party has gained slightly and his personal rating has dropped only marginally.

National leads Labour by 24.6 percentage points - reversing signs of a Government comeback in other polls.

National has risen half a percentage point since June to 55.4 per cent support, while Labour has dropped 1.6 points to 30.8.

The only other party to get across the crucial 5 per cent threshold to enter Parliament is the Greens, with 5.5 per cent. But New Zealand First, at 4.1 per cent, is edging closer.

National would govern alone if the poll were translated into seats. The election must be held by November 15.

A sizeable number in the sample of 770 voters -14.3 per cent - remain undecided about which party to vote for, and that suggests Labour still has an opportunity to claw its way back.

The poll began as truck drivers protested against rising road-user charges and Labour sealed its buyback of the railway company.

Prime Minister Helen Clark's attack on National leader John Key's personal shareholding in TranzRail has had no benefit for her party in the poll, and National appears unharmed by the secrecy over its use of controversial Australian political consultants Crosby/Textor.

The economy was at the forefront of people's minds during the month as economic data revealed a quarter of negative growth and fuelled talk that NZ might already be in a recession.

Asked which issue was most likely to influence their vote, 22.9 per cent said the economy.

A spate of homicides and violence in South Auckland in June was clearly still in voters' minds, as law and order was the second most influential issue (17.2 per cent).

Over the past month National has stepped up its policy releases and revealed a plan to open the Accident Compensation Corporation up to competition, get rid of TVNZ's charter, and introduce a 90-day probationary period for recruits to small businesses.

The major planks of National's election plan are yet to be unveiled, however, and Labour has expressed confidence it will close the gap on its rival once policies are on the table.

Still, the DigiPoll survey will not be welcomed by Labour, which had been heartened by three different polls a week ago showing a narrowing gap. Those put National's lead at between 13 and 17 percentage points.

Helen Clark trails Mr Key in the preferred prime minister ratings in the Herald poll but only just. She registers 44 per cent support, compared with the National leader's 47.1 - similar to the June DigiPoll survey.

Mr Peters has 4.9 per cent backing as preferred prime minister, reflecting his potential to get his party over the 5 per cent threshold.

The poll results have a margin of error of 3.5 per cent.