Bill English has accused Jacinda Ardern of betraying her generation after an iron-clad promise to keep the retirement age at 65 if Labour gets into power.
Asked at Monday's Leaders Debate whether she would resign rather than raise the age, Ms Ardern replied with a steadfast "yes".
"We'll keep it at 65 and we'll start putting contributions back into the Superannuation Fund so we can afford for Kiwis to retire," said Ms Ardern.
Labour argues if affordability is the issue, the Government should start contributing to the New Zealand Super Fund, which it hasn't done for nine years.
Treasury's projected surpluses would be enough to fill the $20 billion hole in the fund, it says.
Former Prime Minister John Key famously made the same assurance - to resign rather than raise the age - but Mr English said Ardern's promise was "letting down her generation".
"They are going to have to pay the bill for that. We've outlined a long-term goal to increase the age of eligibility. We've given everybody 20 years, so today's 25-year-olds know they won't have to carry the burden of those extra two years."
The Government has said it intends to increase the age of superannuation to 67 by 2040.
But the move has left National a party alone, with every other political party in Parliament opposed to the move.
New Zealand First leader Winston Peters has made keeping the retirement age one of his bottom lines. But he backs one part of National's policy - the crackdown on eligibility for immigrants rising from 10 years of having lived in New Zealand to 20 years.
The Maori Party has also backed away from the policy. It wants the retirement age to be lowered to 60 and won't support the Government's plan.
So is there anything Mr English would resign over rather than do? Patrick Gower put the question to Mr English on Monday night.
"I'm not resigning," Mr English replied.