Kiwis are living two years longer now than they did when the age of superannuation was set at 65, costing the taxpayer billions, new figures show.

On average, New Zealand men born today can expect to live 78 years and women 83. 

"That's the median - that means a hell of a lot of people are going to live longer than that," economist Shamubeel Eaqub told The AM Show on Thursday.

The present age of eligibility for superannuation - which is not means-tested - was set at 65 in 2001, when men lived 76 years and women 81.

"Even in the past 20 years, the median lifespans for both men and women have increased by a couple of years," said Statistics NZ population indicators manager Tehseen Islam.

Superannuation is already the biggest chunk of welfare spending in the Government's Budget, and set to rise, with 1.1 million Kiwis expected to be receiving it by 2030. Presently about 780,000 get the payment - more than 15 percent of us.

"It's growing really fast," said Eaqub. "That's going to be the fastest area of growth when it comes to our welfare system - and let's call it what it is, New Zealand super is welfare -  it's the fastest-growing area because it's the fastest-growing part of the population."

Like previous Prime Minister Sir John Key, Jacinda Ardern has said she'd quit before raising the age of eligibility. Her direct predecessor Sir Bill English on the other hand had plans in place to gradually raise it to 67 - matching the increase in longevity since 2001 - but was voted out before he could. 

Ardern has said future superannuation costs can be paid for by investing in the Super Fund, which has had significant returns over its lifetime, and it wouldn't be fair to expect people who have spent their lives doing hard, physical work to continue doing so into their late 60s.

Others have expressed concern over the shorter life expectancy for groups such as Māori.

"I think we have to do it," said Eaqub, who would go further and reintroduce means-testing so it's not wasted on the wealthy.

"We cannot afford for it to be universal forever. When we've got issues with child poverty, the reality is we're saying people who have had the opportunity to work all their lives, own property and build up assets should also get access to superannuation, when kids are left to be in poverty."

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Our increased lifespans also pose a problem for those planning to retire at 65 - if they want to maintain their lifestyle until the day they die, retirees will need to have more in the bank.

"If you're working and saving only up until 65, you're probably planning in reality for about 30 years of retirement," said Eaqub.

That's because although life expectancy at birth is about 81, that includes people who die young - anyone who makes it to 65 has a 50/50 chance of living to the ripe old age of 85, receiving 20 years' worth of superannuation payments.

The Westpac Massey Fin-Ed Centre last year said a city couple would need $787,000 saved to live comfortably in their retirement, while in the regions they'd need $493,000. The Commission for Financial Capability in 2016 came up with a figure of $649,779. Both figures assumed the retiree would own their own home.

Eaqub said $100,000 would probably be enough for a "modest" lifestyle in retirement if you had your own home.