The cost to maintain facilities at most of the country's prisons has risen due to a 10-year half-a-billion dollar contract.
The Corrections Department says the contract covers more services so can't be compared with the previous deal which on the face of it was cheaper.
Its latest annual review shows trans-Tasman company Downer quietly took over the prison facilities contract from Spotless in 2018.
The 10-year contract covers 16 of the country's 18 prisons.
The Spotless contract cost $36 million a year, plus $16m on minor capital works.
Downer's is worth $53m, and also covers the Justice Ministry.
Downer was "providing an enhanced and extended scope of work along with refinements to the Asset Management and Facilities Management responsibilities", Corrections said in a statement.
It covers building and assets maintenance, and waste and hazardous substance management.
It did not include the Downer figure when it reported back to Parliament that it had spent $60m on contractors and consultants this year, up by more than a third on last year, at a time when the government has ordered departments to spend less on contractors.
"We've looked into whether it would be possible to provide the total annual amount spent on multi-year contracts, but this isn't something we can quickly collate," the department said.
If multi-year contracts were included in the annual review figure, it would inflate the spending on contractors significantly.
Spotless had held the prison facilities contract since 2010. Downer took control of Spotless in a hostile takeover in 2017. Downer said it treated the companies as separate entities while bidding for the prison work.
Aside from the facilities contract, Corrections had had to hire "specialised technical expertise and support not typically found within a standard public service agency" to deal with a growing prison population, and to tackle payroll problems, it said.