The rapidly-growing prison population has forced the Corrections Department to consider reopening prison units which were closed just a year ago.
The department is also reporting a trend of more women being locked up, which means some female inmates may have to be housed at a unit in Rimutaka Prison, a men's prison.
Corrections said yesterday it was considering reopening the 112-bed Upper Jail Unit at Rimutaka to cater for an overflow of inmates from nearby Arohata Women's Prison, where an expansion is not yet completed.
The unit was closed just last year because the prison population had been forecast to fall. Instead, the prison population is still growing, and passed 10,000 inmates for the first time in November. The closures were a controversial move, with up to 260 jobs lost - though some prison staff were reemployed at other jails.
The Government plans to spend up to $2.5 billion on new prisons and expanding existing ones over the next few years, but in the short term more prison beds are urgently needed. It is understood another prison unit which was closed last year, at Waikeria Prison in the Waikato, could also be reopened to cater for another 240 prisoners.
"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.