Members must contribute $1043 over the year or around $20 per week to get the full amount but are given 50c by the government for every $1 they put in up to the $1043.
David Boyle, who is in charge of investor education at the Commission for Financial Capability, said it was frustrating people were leaving money on the table.
"It does my head in."
The commission - a crown entity in charge of financial education and reviewing New Zealand's retirement income policy - has campaigned on the "free five hundy"in recent years to try and get people to make the most of the subsidy.
"The good thing is there is a trend of more [people] getting the maximum," he said.
But the number of people not getting the subsidy at all was worrying and it was growing in numbers.
The percentage of those not getting any subsidy has risen from 27 per cent to 28 per cent in the last three years - an increase of 106,502 people.
Boyle said most of those not getting any subsidy were probably non-workers, either spouses that were not employed or students.
People not working are still eligible for the subsidy but they need to contact their KiwiSaver provider directly to organise making a contribution into their account.
A direct debit of $20 a week was enough for people to get the full subsidy, Boyle said.
If they don't know who their KiwiSaver provider is they can find out by contacting the IRD.
Boyle said some of those not getting the full subsidy would be earning less than $35k a year.
"If they are only doing 3 per cent they will not be getting the maximum."
Boyle said those people could help top up their account by increasing their contribution rate to 4 per cent by talking to their work payroll department or by making extra top-up contributions directly to their KiwiSaver provider.
While for some the extra money would be unaffordable others were simply missing out because of apathy, Boyle said.
"It is easy to forget about it because it is not top of mind," said Boyle.
But he said members should keep in mind what they are missing out on.
Over a working lifetime (18 to 65) the subsidy would add up to $24,487 and that's not including investment returns on the money or compound interest.
"Can you afford not to do it?"
How to get your $521
• Ensure you are putting in a least $20 a week to your KiwiSaver account
• If you earn under $35k, increase your contribution rate from 3 to 4 per cent
• Check how much you have contributed with your provider and do a one off top up before June 30 to reach the $1043 minimum
