"Corrections must at all times ensure it has enough beds of the required standard across the prison network," Deputy National Commissioner Rachel Leota said yesterday.
"Although more beds have been added, the female prison population in particular has risen, necessitating additional beds."
The rise in female prisoners has also forced Corrections to consider alternatives to imprisonment. Leota said the department would consider increasing the use of electronic monitoring of offenders, rather than locking them up.
The placement of women inmates in a men's prison at Rimutaka could raise concerns about prisoner safety. But Leota said the unit was geographically isolated from the main prison and inmates were "unable to see each other".
"The unit will be brought up to standard and more staff brought in to ensure the ratio of prisoners to frontline and support staff is maintained," she said.
"We take the safety of prisoners, staff and the public extremely seriously and these changes to capacity will not compromise the safety of staff, prisoners or the security of our sites."
The costs of reopening the units at Rimutaka and Waikeria are not known. But when they were closed along with a unit at Tongariro-Rangipo, the estimated savings were $20m plus up to $145m in capital costs.
In July, Corrections announced it would increase capacity at five prisons - Arohata, Whanganui Prison, Hawke's Bay Regional Prison, Mt Eden Corrections Facility and Christchurch Men's Prison - partly through double-bunking.
In October, the Government announced further expansions, including a new 1500-bed prison at Waikeria, at a cost of up to $2.5 billion.
The rising prison population has been driven by tougher bail laws, stricter sentencing and repeat offending by drug and alcohol addicts.