New Zealand's Retirement Commissioner has hit back at criticism of the country's pension system and lack of reform.

Last week a visiting expert said New Zealand was lagging behind other countries by failing to under take a comprehensive review of its retirement policy.

That followed a report released by the Organisation for Economic Cooperation and Development (OECD) which pointed to New Zealand being one of the least generous nations when it came to comparing the state pension to the average wage.

New Zealand was ranked sixth worse equal with Australia in its 2017 Pensions at a Glance report with just 43 per cent of the average wage being paid out to retirees.

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That compared to the top OECD ranked nation Norway which paid out 110 per cent of the average wage.

Diane Maxwell, said tables like the one in that report were a simple way of comparing countries but it wasn't that simple.

"It is a tidy equation to use to simplify something that really isn't that simple."

Maxwell said looking at the replacement rate - how much people get when they retire compared to when they work - was not that helpful for people.

"It often bears no resemblance to what you spend while working."

"Working is expensive."

Using herself as an example she pointed to the costs of getting to work - driving a car and paying for petrol and parking as well as the cost of childcare and doggy daycare and paying someone to do the housework and gardening.

"The day you stop work is the day you start doing your own garden and washing your own car," she said.

In the United States, where she has just visited for a major pension policy conference, Maxwell said people were told they needed to have 75 per cent of their income to live off in retirement.

"You do not."

"In reality it is a very individual thing."

She says nice tidy charts out of the OECD don't allow people to compare the wealth and wellbeing of people and information like whether healthcare is publicly funded, there are free buses and rates rebates - all factors which New Zealand has.

"They matter greatly for retirees."

"It is not about just the relationship to what you earned but the cost of living."

New Zealand has also come under fire in the last week from a visiting pension expert who said country was lagging behind when it came to global changes.

David Harris, a consultant based in the United Kingdom, said the country was getting out of kilter with the rest of the world where many had moved to increase the retirement age and push up savings rates.

Raising the age of eligibility for New Zealand Superannuation and increasing the contribution rate for KiwiSaver were both recommendations made in Maxwell's 2016 review of retirement policy.

The previous National-led government rebuffed her proposal to increase the KiwiSaver rate from 3 per cent to 4 per cent but had planned to raise the retirement age from 65 to 67.

But new Labour Prime Minister Jacinda Ardern has already said she will not raise the retirement age under her leadership although the Government is considering an increase to the KiwiSaver rate.

Maxwell said New Zealand needed to raise the age of eligibility for New Zealand Superannuation.

"To think you can be retired for 30 years and still get super at 65 ... it's not affordable. We are going to have to raise the age."

Asked what she planned to do in the face of Ardern's stance Maxwell said: "My job is to keep being an independent voice."

Maxwell said it was not easy to see that the population was ageing.

"But it is happening."

She said New Zealand Superannuation is costing $38 million a day at the moment but in 20 years time it would be $117 million a day.

"It is terrifying."

Maxwell said 65 today was not old for many people. While there was a group of people who would not cope with the age being raised, 65 was already too late for many of them.

She said the Government needed to step in and help older workers retrain and for those who could not work due to health, provide a benefit to them which was not New Zealand Superannuation.

Half of the OECD countries have already raised the retirement age.

"We can not pretend this is not coming down the track."

Maxwell said people needed a long lead-in time so they could plan for the change.

She said the alternative was increasing taxes or reducing other areas of spending like health or education to keep paying for it.

"Our politicians need to be our leaders. They need to lead us through difficult decisions and an ageing population and how we deal with it is one of those."