An internal Corrections email has revealed the department is looking to hire hundreds more staff than it has publicly disclosed.
The email from Corrections national commissioner Jeremy Lightfoot said the department would need 800 extra staff by September next year.
That's the equivalent of 10 percent of the Corrections workforce, and is 200 more than Corrections had publicly said it needed.
Last year, citing reduced reoffending, Corrections cut nearly 200 jobs and closed several prison units.
Now it is looking to reopen those same units to cope with an expected increase in the prison population.
Green Party corrections spokesperson David Clendon said the department couldn't make up its mind whether it should be hiring or firing.
"We don't know what forecasting they're basing this on, but it does seem to be absolute confusion within Corrections as to what their staffing needs are.
"The constant hiring and firing of officers would have to make people think twice about their job security and the committment of Corrections to its staff, their willingness to cut staff quite dramatically against the advice of the Corrections Association."
Corrections' chief custodial officer Neil Beales said changes to the Bail Act had led to the prison population increasing beyond expectations. The prison muster had passed the 10,000 mark and the increase was not slowing.
"It looks like we're going to have to consult on reopening some previously closed units, such as the Nikau unit at Waikeria or the top jail at Rimutaka.
"That is subject to consultation - it hasn't been finalised at this stage - but if that does indeed happen then we're going to have to look at more staff than we'd previously planned."
Asked why Corrections has gone from closing units and laying off hundreds of staff last year to reopening them and hiring many hundreds next year, Mr Beales said prison population forecasts were "more of an art than a science".
He said Corrections relied on Ministry of Justice prison population forecasts.
"One of these things that you can never predict is changes in legislation ... that coupled with changes to the Bail Act, so what we've seen is a rise in the remand numbers, and that rise continues."
"And we've seen people now staying in prison for longer, and doing longer sentences."
Mr Beales could not say exactly how much the campaign to recruit 800 new staff would cost, but said "it isn't cheap, let's be honest".