The Corrections Department has spent $12 million fixing its payroll system and reimbursing staff, $4m less than it expected.
The prisons operator is one of thousands of private and public entities estimated to owe workers $2 billion for breaches of the Holidays Act.
"The total payments we have been required to make to bring Corrections in line with the Act have been significantly lower than initially forecast," Corrections said in an Official Information Act response.
It had paid back all 8000 current staff by late last year and planned to do the same for 10,000 former staff by the middle of this year, it said.
The arrears amounted to $7m, and the remaining $5m was spent fixing its "specialised and complex" payroll system.
Meanwhile, the government's review of the Holidays Act is dragging on, even as the bill for breaches of the Act mounts.
It is almost two years since an independent taskforce began reviewing the law, and its report back to the government was delayed by two months last year.
"There is no hold up. There was no deadline," Minister of Workplace Relations Iain Lees-Galloway said in a statement.
"We are preparing our response to the report and expect to make that public soon."
Some payroll companies have said they were shut out of the review.
The taskforce was asked to make recommendations for transparent rules for providing entitlements to, and payment for, holidays and leave.
It did not look into how to fix payroll breaches. That work has been tortuous for some entities, including the country's district health boards which owe more than 100,000 workers more than half a billion dollars.