Kiwis have overpaid their student loans by more than $100 million in the past five years, according to data released under the Official Information Act.
The figures show Inland Revenue has paid $119,817,880 in refunds to borrowers who have repaid their loan in full since 2012, with 243,940 people getting money back.
Inland Revenue student loan business manager Maurice Lawlor said borrowers may end up paying more than they owe if they continued to use their student loan tax code or did not stop their automatic payments in time.
"A refund is usually issued when the loan is closed, but in some cases a refund cannot be issued at the time because we do not hold current contact or bank account details."
A scheme had existed where borrowers could receive a voluntary repayment bonus if they paid more than required towards their loan, although this was scrapped in 2013.
The 2012 tax year saw the highest value of refunds and number of borrowers receiving them in the past five years, with 67,276 people getting $28,933,519 between them, which Lawlor said may be attributed to the repayment bonus scheme.
Since the scheme ended, refund amounts have steadily dropped, from the 2012 high to $19,170,298 in 2016.
New Zealand Union of Students' Associations president Linsey Higgins said the millions refunded after overpayments is a result of an "inadequate system" that is in need of a "giant overhaul".
"It's another sign that Inland Revenue doesn't have the system it needs to do the job properly," she said.
"Inland Revenue should have a system that doesn't allow for such significant overpayments, we know the system is woefully inadequate."
She said a lack of clear information was causing confusion among borrowers, and was leading to overpayments and disputes regarding owed amounts.
"I think it is quite vague, unless you deliberately go online and watch your account like a hawk, you only see your loan balance once every six months, and in a six-month period your loan balance could change significantly.
"It's about highlighting when a loan balance gets quite low and saying you might need to change your tax code soon. Otherwise, you just forgot about it and you know 12 per cent of your pay cheque is going off somewhere."
Earlier this month a woman was arrested for defaulting on her loan repayment, as she tried to board a flight to Australia at Auckland Airport, and subsequently appeared in Manukau District Court.
In January, as Cook Islands man Ngatokotoru Puna, 40, tried to leave New Zealand, he was arrested for not repaying his loan.
There are 111,487 borrowers living overseas who collectively owe $3.2 billion, with 86,185 of those in default owing $995m in overdue payments.
An IRD spokesman said a "number of campaigns" were designed to identify and contact overseas borrowers, but many don't keep their contact information current.
"We use direct marketing by email and text message. Inland Revenue works with third-party providers to track and trace borrowers overseas to get their most recent contact details.
"So that overseas-based borrowers can contact IR easily, we have toll-free calling options from Australia and the UK and an email address to access our collections teams."