Corrections has had to put the brakes on the closure of parts of Waikeria Prison because prisoner numbers are too high.
Corrections boss Ray Smith has told a select committee that the prison muster has grown faster than expected and they can't cope without the beds at Waikeria.
"It's hard work, there's no doubt we're busier, but it's a good busy," he said.
Mr Smith said there were 9300 prisoners across the facilities in New Zealand, with about 1000 beds spare.
"As you get further into that you have to start using the spaces that are not as desirable."
Mr Smith said overall Corrections was managing with the space available.
"I think the facilities can stretch but we need to make sure we have enough staff."
Corrections also had to refurbish parts of Springhill Prison after the riots there.
The department's been under fire after widespread issues at Serco-run Mt Eden -- which both Corrections and Serco blamed on high prisoner numbers.
Ongoing battles at the remand prison are being blamed on a lack of staff.
Labour MP Phil Goff said he has figures which show there are four inmates per staff member at Mt Eden, while the average across other prisons is fewer than three prisoners per staff member.
Mr Smith visited Mt Eden on Friday and is defending its operation.
"I'm very confident with the way it's being run now."
Mr Smith says things have improved since he stepped in and took control of the prison from Serco, and the private company has been disciplined for their failures.
"They have been held to account by me."
Serco is currently challenging a Chief Inspector's report into fight clubs at Mt Eden.