Prime Minister Bill English has hit back at opposition parties for claiming the government's plan to raise the retirement age will come at a cost to younger generations.

Mr English announced on Monday the age of eligibility for national superannuation would go up from 65 to 67 in 2040, with the change starting in 2037.

It's been attacked by Labour, the Greens and NZ First, and there have been warnings of "intergenerational warfare" because it's the younger generations who will face having to work for longer.

Mr English said there's "a fundamental misunderstanding" about the impact of the proposed changes.

"The decisions we are making will reduce the burden on future workers, the Greens, Labour and NZ First positions will maintain the burden on future workers," he told reporters on Wednesday.

"It's appropriate to reduce the cost of superannuation by $3 billion or $4b by 2040 - that means less tax will be required from those who are working... so it's complete nonsense for other parties to say we are increasing the burden."

Mr English said he believes most people see the sense in the move.

"I think most people think that raising the age was something that was inevitable. This is probably a good time to do it. I've been pleased with the broad public support for it, it's really just the political parties, who now look a bit at sea, who are upset about it."

Mr English intends introducing a bill next year to put the changes into law.

But if National loses the September election that won't happen.

And if it wins, it's doubtful he will have the numbers to pass it because all the other parties oppose the plan.