New Zealand's conservative Prime Minister Bill English had raced to a strong lead midway through election counting Saturday, although the figures still gave opposition leader Jacinda Ardern a realistic prospect of forming government.

Central to the vote count was the performance of maverick Winston Peters' New Zealand First (NZF) which loomed as the likely kingmaker.

With 50 percent of the vote counted, support for English's National Party was 46.7 percent, while Ardern's centre-left Labour was on 35.5, with NZF at 7.3 and the Greens at 5.9.

The major parties must forge coalitions to reach a majority under New Zealand's proportional voting system, meaning English would still need support from NZF to form a government.

English, with support from current ally ACT, would have 59 seats, two short of the 61 needed to win.

But senior National MP Gerry Brownlee cautioned: "you wouldn't want to go speculating at this stage. We'll just wait a bit of time yet."

Ardern's Labour Party would need a lift in votes in the latter stages of the count as her combined seat tally with the Green and NZF would only be 59 at the halfway stage.

National campaign director Steven Joyce said the early figures were encouraging for English but it was far too early to see how the election would play out.

The early results are mainly advance votes cast in the two weeks before election day and have historically tended to reflect the final numbers.

"I would like to see Labour a bit stronger than the numbers that we're showing," Labour MP Phil Twyford told Radio New Zealand.

"But Labour is back under Jacinda's leadership. She completely electrified this campaign and turned it into a very tough two-horse race."

The campaign has been the most volatile in recent memory, with momentum swinging from English to Ardern and then back again. 

No party has claimed a majority government in New Zealand's 120-seat parliament since proportional voting was adopted in 1996 and this election is unlikely to change that.

Peters, a populist anti-immigration campaigner, had given no indication of which party he would support and told reporters after the polls closed that he had seen "too many campaigns for nerves".

Ardern was hoping for a high youth vote to counter her dip in the polls in the final week of campaigning and visited universities across the country encouraging students to cast their ballots.

 - 'Jacinda-mania' -

English's National Party was in the driving seat to win a fourth term until Ardern took over the Labour Party last month. 

The 37-year-old galvanised support for the ailing centre-left party, giving it a 20-point popularity boost to bring it level with National.

Ardern accused the government of inertia, saying that after three terms it has run out of ideas on issues such as affordable housing and protecting the environment. 

Her policy platform includes free tertiary education and slashing immigration to reduce pressure on housing and infrastructure.

Ardern is bidding to become New Zealand's youngest leader since 1856 and only the third woman to lead the South Pacific nation of 4.6 million people. 

But the "Jacinda-mania" phenomenon waned as English attacked her financial credibility while pointing to his economic record over the past nine years.

The 55-year-old ex-farmer and father-of-six, who took over as prime minister when John Key stepped down last December, argued only National can maintain strong economic growth. 

English also wants to make amends for his last leadership foray in 2002, when National slumped to a record defeat and won barely 20 percent of the vote. 

While tipping a close race, he is confident National can win a fourth term, a feat no New Zealand government has achieved in more than 50 years. 

The wildcard is Peters, a 72-year-old political veteran who has shown in the past that he will back either side if the right offer is made. 

In 1996, he helped install a National-led government in return for being made deputy prime minister, then in 2005 he joined a Labour coalition after being given the job of foreign minister.

Source: AFP