"We needed to reconsider the balance between savings and investment and technological change," says Cullen. "One of the ways of doing that was to try and drive stronger New Zealand financial markets which over time would then be able to invest more in New Zealand and produce more of our own savings."
The early versions of KiwiSaver included a $1000 government kick-start and $40 fee subsidy, as well as the ability to divert some of the money into a mortgage.
Six weeks before its launch, Labour pumped up the incentives by introducing a dollar-for-dollar annual subsidy, called the member tax credit, capped at $1043 a year.
Cullen says much of KiwiSaver's design was weighted in favour of those on low to middle incomes. "It was very much designed to tilt the playing field towards those with modest incomes."
To that end, the government contribution was capped, so people with bigger incomes didn't get more from the public purse.
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The total return was designed to be a higher proportion for those who didn't earn as much. "That was fundamental."
But then, Labour lost the 2008 election.
Since then, the National-led Government has progressively watered down the incentives - dropping the kick-start and fee subsidy, halving the government's annual tax credit to a maximum of $521 and taxing employer contributions.
Despite all the changes, Cullen believes the scheme has been a success so far.
"It has succeeded beyond our projections," he says.
However, he would do some things differently.
"I think in retrospect, it would have been better to limit it to over 18 or the point of entry to the labour market."
Without that limit, he says, many well-off people used KiwiSaver to get $1000 for their kids.
As of June 30 last year, 352,000 under 18-year-olds were signed up to the scheme, although the number of new sign-ups has fallen since the $1000 incentive was removed.
Cullen also believes the Government should undertake a one-off auto-enrolment and make the KiwiSaver scheme compulsory.
"On balance, I would be moving to compulsion now."
About 540,000 people aged between 18 and 64 are not in the scheme.
Cullen says that while some of those would be well-off people who don't need KiwiSaver, others are low income earners.
It's about getting people into a savings habit.
"It does worry me that a lot of the people in that group are probably on low incomes."
He says those who don't join the scheme will be considerably worse off than their peers who do join.
"That is quite a worry when it comes to long term equity, particularly with more people ending up in retirement without a home or a mortgage-free home.
"That is an important social consideration."
Cullen says the Government should look at a phased-in approach, where people can start contributing at 1 per cent of their income and move up to 3 per cent over time.
"It's about getting people into a savings habit," he says.
While getting those people on board now might cost a bit, if they don't join and end up relying entirely on New Zealand Superannuation and renting in Auckland, that could cost a lot more, he says.
"One of my biggest criticisms of the current Government is the lack of long-term thinking."
And Cullen believes KiwiSaver can help address one of the big issues facing New Zealand - falling home ownership.
"There is a big wall facing us at the moment on the housing side," he says.
"If we don't do better, then we are going to face for the first time in history [in New Zealand] a substantial portion of people reaching retirement without their own home, mortgage free or nearly mortgage free."
He says KiwiSaver can give people the chance to buy a home - if not early on, at least before retirement.
Then there are all the people who are in KiwiSaver, but not contributing to it.
In the year to June 30 2016, about 580,000 people received no government tax credit - meaning they did not contribute a single dollar to KiwiSaver.
"What we don't really know is, what is the makeup of that group? Why is it they are not contributing?"
Cullen says more needs to be done to answer those questions before a solution can be found.
"It is not satisfactory to have apparently 2.7 million people in KiwiSaver but a great deal less than that actually contributing."
But he points out that KiwiSaver is still young, even after 10 years.
"There is an awful lot of maturation to come."
He says people will have to be contributing for 40 years or more to really know how it is going.
The average balance is only about $14,000 to $15,000, which seems low after 10 years.
But he says even those numbers suggest the average balance will be in the six figures by the time people have been in KiwiSaver for a lifetime.
"I think you will see those numbers become much more significant over the longer term."
A worker who earns the average wage and gets 6 per cent added to their KiwiSaver - from their own and their employer's contributions - will see a significant boost in security in retirement.
"It means if they need a knee replacement they can afford to go and get it without waiting.
"Those things increase the quality of life."
So was the scheme's designer one of the first people to sign up to KiwiSaver? No, answers Cullen.
"It was not designed for people like me," he says.
Cullen was 62 when the scheme launched.
"I was close to retirement, on a high income and I was one of the lucky ones."
As an MP, he was part of the parliamentary superannuation scheme - a defined benefit scheme which stopped taking new members after 1993.
Now 72, he still has a substantial income in retirement. As well as his pension payments, Cullen has roles on several boards and advisory positions.
