Christopher Luxon reckons Labour should be endorsing National's tax plans rather than finding holes in the party's costings.
But Labour leader Chris Hipkins has instead gone on the attack, questioning the lack of detail in National's policy.
Since National released its tax policy two weeks ago, Labour has been going through the document looking for any holes or errors it can find with how the party expects to pay for the tax relief it is offering.
That has included claiming New Zealand's double tax treaties will restrict its ability to introduce a foreign buyers' tax; National's assumptions about how many foreign buyers may be taxed are heroic; and National will have to slash money from services like passport processing and emergency management.
National has rejected these allegations, saying it has advice showing its plans will work and that it will ensure there is no money taken out of frontline public services. While the party wants to reduce "back-office expenditure" by an average of 6.5 percent, it's excluded some departments from having to make cuts to the frontline - like in health and education.
Speaking on Wednesday, Luxon said there was a "range of opinions", but he believed National's figures are "absolutely rock solid".
"I am absolutely confident and rock solid we know what we are doing and we will be able to deliver this," he said.
He initially said Labour and the Council of Trade Unions (CTU) - which has also been attacking National's plans - should be "endorsing this" package.
"Why? Because we have developed a programme and a plan that gives relief to low and middle-income New Zealanders. I want to be on the side of working New Zealanders and that's what this tax plan does."
He went on to say Labour was "disappointed" National is "delivering for working people".
"I think it's pretty sad but I would expect the CTU and I would expect Labour to come and say, 'Good on you, you've actually found a way to deliver for working New Zealanders who are doing it incredibly tough.'"
National's tax relief package includes shifting income brackets to account for inflation, introducing a new childcare tax credit and increasing Working for Families tax credits. An average household with children with an income of $120,000 would get up to $250 a fortnight.
Hipkins continued to criticise National for not answering questions about its plan.
"We are not talking technical numbers here, we are talking billions of dollars they cannot account for," the Labour leader said on Wednesday.
"The removal of the foreign buyer ban simply doesn't stack up. They haven't found anybody that has argued that will generate the sort of revenue they think it will.
"Their increase in gambling also doesn't stack up and nobody believes that's credible.
"Their public service cuts, which would be very significant, they are refusing to say exactly what they would cut, or how deep it would be, or what services are going to be affected or removed as a result."
Labour's proposal to help with the cost of living includes cutting GST from fresh and frozen fruit and vegetables. It has already removed the prescription co-payment and reduced public transport fares for some groups.
