Labour leader Chris Hipkins says National should never have promised to fund specific new cancer drugs, and that a lack of funding for the pledge in the budget is "just cruel".
But Finance Minister Nicola Willis says the government will deliver on its promise.
National promised during the election to fund 13 specific cancer treatments at a cost of $280 million - paid for by reinstating the $5 prescription fee for some people - from 2024/25.
While this year's Budget confirmed the change to prescription fees, there was no funding for the cancer drugs.
Finance Minister Nicola Willis over the weekend said an announcement would be made about funding for the drugs "very soon", and Prime Minister Christopher Luxon on Tuesday morning confirmed that would be before the end of the year.
"It has required more time than we anticipated and we're just working out the best way to procure those treatments," Luxon said.
"We're working really hard to make sure when we put the process together to secure those drugs and treatments, that we've actually got it set up on a sustainable ongoing basis, that it actually works."
Hipkins, speaking to reporters at Wellington's Sustainability Trust, said National should never have announced the policy in the first place.
"Being that specific in terms of what drugs should and shouldn't be funded, I don't think politicians should be doing that in an election campaign, I think it's wrong," he said.
"They do do this in other countries and then it becomes really ripe for the pharmaceutical lobby to get in and the criteria for access to life-saving medications becomes more than just the scientific and medical criteria, it becomes political access and all those sorts of things, and who can run the best PR campaign.
"For someone who claims to be a commercial negotiating genius, Chris Luxon seems to have missed a fairly obvious fact, that when he named 13 cancer drugs and said 'these are drugs we're going to fund' his negotiating power was somewhat diminished."
He said that having made the promise to New Zealanders, however, the government should have then followed through.
"For some people that literally is a matter of life and death ... I don't think they should have gone down that road, but having done that - to promise it and then not to deliver it is just cruel."
He said Willis' argument the Budget had needed reworking because of unexpected expenses - including what she said was a much larger cost to continue Pharmac's general funding into the future - was "just absolute nonsense".
"It's very clear on a day-to-day basis now that the National Party - Nicola Willis in particular - was either too lazy or too incompetent to actually read the last Budget. All of the things she's claiming like fiscal cliffs and so on - the time-limited funding initiatives that we put in place - she's now created more of them in this Budget.
"There were no nasty surprises there, it was all transparent.
"There's an awful lot of things they could have funded if they hadn't given $2.9b worth of tax cuts to landlords - that was their first priority."
He noted National's own fiscal plan had included funding for the free school lunches programme and for Pharmac, "so they knew that was something that a future government was going to have to make sure it funded, so this is just Nicola Willis not being able to make her numbers add up".
"You'll have lots and lots of quotes from them saying 'we know numbers'. Turns out they don't."
He said Luxon's commitment to make an announcement about the drugs by the end of the year did not match expectations it would be funded by 1 July, "and as I said for the people who are awaiting that, it's a life and death situation".
Hipkins also criticised the tax cuts, saying New Zealanders had high expectations from the "big, big promises" of National who he accused of being "out and out dishonest" in claiming the average family could get $250 a fortnight from tax cuts, rebates and other support.
"It was never going to be that. Very very few families would ever qualify for that full amount and the National Party were never clear about that during the election campaign.
"The very first example in their tax flyer for example suggested that a family with one income earner with $60,000 and another with $50,000, two children in early childcare education and two other children - you'd be talking about a handful of families across the whole country that would be in that category."
He said the FamilyBoost childcare subsidy - which was also shifted after the election to require parents to submit receipts every three months - would be a "nightmare for families to claim" and he believed only a fraction of eligible families would actually receive it.
"Again, it's another situation where you have to tick a lot of boxes to get the maximum subsidy that the National Party talks about ... they've gone big on the spin and very light on the substance."
Willis confirms intention to fund cancer drugs
In a speech to business leaders in Canterbury, Willis said she had known National in government would need to find about $700m over the next four years for Pharmac.
"What we didn't know was that continuing to fund the medicines that New Zealanders rely on was actually going to cost an additional $1b on top of that.
"Both National and Labour campaigned on finding more funding for Pharmac, but neither party campaigned on finding $1.8b."
She said the government had to address the Pharmac shortfall first, but would meet National's commitment to fund the 13 cancer drugs.
"I can assure you we will meet that commitment ... we will do so with urgency and I expect us to have more to say on it soon."
She told reporters afterwards there was still some complexity about how the medicines were approved and procured and the order in which that could happen.
"I note that there is no other political party that is prepared to make that commitment. We made that commitment and we're prepared to deliver on it."
She said tax relief would also make a difference to many families.
"I think we've done a really good job of targeting tax relief to lower and middle income families."