The eligibility age for superannuation will rise to 67 years old by 2040, Prime Minister Bill English revealed this afternoon.

In a major political development, English promised to begin progressively lifting the threshold from 65 to 67 years old in 2037.

That means the changes will not affect anyone born on or before 30 June 1972.

The Government will not act on the promise until after the general election in September. English said the major change would be legislated for next year.

"New Zealanders are healthier and living longer so adjusting the long-term settings of NZ Super while there is time for people to adapt is the right thing to do," English said.

In justifying the change, English noted that even someone who retired at age 67 was likely to receive NZ Super for longer than someone who retired at age 64 today.

"That is because average life expectancy is increasing by about 1.3 years each decade."

Earlier, in a series of difficult interviews this morning, English promised to confirm his plans for superannuation "quite shortly" but did not specify a date.

He now appears to have been pressured into releasing his plans today.

Any changes to NZ Super will be highly sensitive, let alone in election year, and English has been urged by Opposition parties to "come clean" about his plans as soon as possible to give the public some certainty about their retirement.

English said on Saturday the Government was "working through the affordability" of the New Zealand Super scheme.

Unlike his predecessor John Key, he had not undertaken to leave superannuation alone and that meant the Government had an opportunity to consider a "reset" of the pension rules.

He has since ruled out means-testing for NZ Super, changing the way payments are made, or lowering the eligibility age to 60 for Maori and Pacific people.

Experts have said the Government will have to do more than raise the age of eligibility to keep the Super scheme affordable.

The cost of the scheme is expected to triple in the next 20 years from $11 billion to $36b as more people reach the over-65 age-group and live for longer.