"These new monthly membership growth numbers, averaging 12,000, fall short of the average 15,000 per month recorded for the first ten months of 2012."
IRD's most recent KiwiSaver Annual Report showed membership grew on average by 25,000 individuals a month last year.
Stubbs said it was natural that membership growth was slowing down as KiwiSaver was now reaching a point of maturity.
"There are just fewer people who are becoming eligible to join, it's as simple as that. I would call it a sign of success, actually."
Personal finance writer Mary Holm agreed that the slowdown in membership growth was to be expected because there were only so many people who wanted to belong to the scheme.
"All the keen ones are already on board," she said.
But she said there were people who could join but chose not to.
According to IRD, about 67 per cent of eligible people aged 18 to 24 years and 29 per cent of eligible children were enrolled as of June 30.
"A lot of people say they can't afford to and in some cases I challenge that but there are clearly some families who just really struggle and feel they can't afford it," Holm said.
Most other reasons people gave for not joining "don't hold up" when challenged.
One of the key features of KiwiSaver is auto-enrolment of employees, meaning a person entering the workforce for the first time, or starting with a new employer, is automatically enrolled into KiwiSaver.
There are six 'default' providers for those employees who do not actively select a KiwiSaver provider.
Tower, one of the default providers, believed the current automatic KiwiSaver enrolment system had been a great success early on.
But Stubbs said it could be time to consider "fine tuning" the design of default funds.
With the default providers' terms of appointment due to expire on June 30, 2014, the Ministry of Business, Innovation & Employment (MBIE) is looking at whether the current default arrangements are still suitable.
It last month released a discussion document called 'Review of KiwiSaver Default Provider Arrangements'.
Stubbs said he was keen to see changes made and that Tower "welcomes public discussion on how to improve the KiwiSaver system".
He wanted to see new members automatically placed in a default fund, not just a default provider, that best suited the risk profile of their age.
Younger new members "who should usually be taking more investment risk" would be placed in growth rather than conservative funds, he said.
"As they grew older, these members would be migrated automatically at intervals into the next life cycle fund most appropriate to their changing risk profiles."
Opt-out options would be needed for such people as those who were saving up through KiwiSaver for a home purchase deposit.
The MBIE's discussion document is open for submissions until Christmas Eve.
