The opposition's flailing tax plan was again centre stage of New Zealand's election campaign in the first major debate ahead of the October 14 poll.

The economy-focused ASB Great Debate in Queenstown on Thursday night was a combative four-way tussle, with sledges from all sides.

Finance Minister Grant Robertson represented Labour and opposition finance spokeswoman Nicola Willis flew the flag for National, with the pair joined by minor party leaders David Seymour, of right-wingers ACT, and the Greens' James Shaw.

Unsurprisingly, the number one issue of the campaign debate - National's $NZ14 billion ($A13 billion) tax plan - drew the most heated moments.

Earlier on Thursday, an independent group of economists released a damning report into one element of the plan - a 15 per cent tax on foreign buyers of homes valued at more than $NZ2 million ($A1.8 million).

The last Labour government banned foreigners - exempting Australians and Singaporeans - from buying homes in NZ.

National says the new tax will help pay for income tax cuts, but it appears to have wildly overestimated its forecast take.

The economists, including right-leaning former Reserve Bank economist Michael Reddell, believe it will bring in just $NZ210 million ($A193 million) each year rather than the $NZ740 million ($A680 million) National has budgeted for.

"I'll tell you about economists," Ms Willis said.

"Yyou get six of them in a room, there'll be seven different opinions.

"I stand by our costings."

Labour, which has fallen behind in the polls, believes it is onto a winner with the costings issue and has demanded to see National's workings for days.

"If you're so confident where are the actual costings?" Mr Robertson thundered.

"Release the costings."

Mr Shaw might have summed up the debate best.

"I don't think that National's numbers add up, and I frankly don't think a lot of people care," he said.

"They just want a tax cut."

Mr Robertson's record of six years in office came under fierce attack from the opening question.

The Labour veteran defended extensions to his own operating allowance that have blown out a return to surplus to 2027.

"We have to respond to the events that are in front of us and that's what happens with allowances," he said.

"I hear a lot of professors of hindsight economics up on the stage here who seem to think that we didn't go through COVID.

"We did - and most parties see that that time said 'let's make sure we look after people'."

Mr Seymour was quizzed on ACT's alternative budget, which would shrink the public service by almost a quarter, from 62,000 employees to 47,000.

"People think this will make the ACT party unpopular in Wellington, but we get a lot of people within the public service say 'I'm voting ACT because I hope you fire the guy in the next cubicle'," he said.

Mr Seymour and Ms Willis won the debate based on the response from the business-friendly crowd, with Mr Shaw booed when asked about his wealth tax.

However, all enjoyed laughs and applause at different moments in a free-wheeling debate held in the spirit of Kiwi campaigning, with participants happy enough to share a group picture afterwards.

The stakes will be raised on Tuesday when Prime Minister Chris Hipkins and Opposition Leader Chris Luxon meet in Auckland for the first leaders' debate.