Simon Bridges expects Labour to reveal tax hikes "on the 20th of September" if they win the election held the day before.
"There's so much debt that they're piling on New Zealanders, I don't know what else they would do to pay it all back," the former National Party leader told The AM Show on Friday.
The Government has borrowed tens of billions of dollars to ease the country through the economic impact of the COVID-19 pandemic.
While some reports have had economic activity largely back to normal now the country is free of community transmission of the virus, others suggest when the wage subsidy expires, unemployment will spike.
"The scheme is now simply an expensive exercise in delaying inevitable job losses," Infometrics forecaster Gareth Kiernan said on Thursday, tipping unemployment to hit almost 10 percent next year.
Labour MP David Parker, Associate Minister of Finance, said he's heard nothing from his boss Finance Minister Grant Robertson about tax hikes.
"I expect I would know if it was being considered - so I'm saying 'not that I'm aware of'... I think there are always ways to improve the tax system. At the moment it's an imperfect system, but I don't think we're going to be running on tax at this election."
He said Labour wouldn't "necessarily" put taxes up to pay back the borrowing, but hinted it may depend on who they're in coalition with, should they be re-elected.
"Not necessarily. If you mean the middle class who already pay a high rate of tax, then no. They're not the people who get away with low rates of tax.
"We took a crack at introducing a capital gains tax during this term in Parliament. We couldn't get it across the line for people's untaxed income, and Jacinda made the promise she wouldn't do it... I'm wary of taking off-the-cuff decisions on TV about tax."
NZ First scuppered the planned capital gains tax.
Bridges said Labour is keeping their tax moves "in the safe".
"They'll bring that out on the 20th of September."
In contrast National is planning "mildly radical" moves, new leader Judith Collins said last week, including potential corporate tax breaks and a dramatic halt in spending.
National finance spokesman Paul Goldsmith said last week he'd try and get core Crown debt below 30 percent within a decade, "give or take a few years".
Robertson told Stuff that would result in "some of the deepest cuts to public services in New Zealand's history".
"We know from what happened in the late '80s and early '90s in New Zealand is the impact that that kind of austerity can have."
Before COVID-19, Crown debt was low by international standards at around 19 percent of GDP. It's tipped to rise beyond 50 percent by 2023, similar to where it was in the early 1990s when then-Finance Minister Ruth Richardson slashed spending - social welfare in particular - and sold off assets.
