More than 100,000 New Zealanders have overpaid a total of $6.4 million on their student loans.
The system has been criticised by a students association, which says graduates are overpaying their loans because the system is too hard to understand.
Once the Inland Revenue Department sends them a letter telling them their loan has been fully repaid, borrowers have to change their tax code, stop any extra deductions being made by their employer and cancel any automatic payments.
Inland Revenue figures, released to the Herald under the Official Information Act, show most borrowers who have overpaid have "very small" credit balances - the median balance is $2.57 - and 76 per cent of borrowers (78,280 people) who have overpaid did so by less than $20.
But while Inland Revenue would not give details on the top 10 amounts that people have overpaid, it said all were more than $11,900 in credit.