Corrections Minister Judith Collins says she released information showing the number of prisoners in New Zealand was at the highest level ever because the public had a right to know.

Ms Collins announced there were 8509 people in prisons or police stations nationwide yesterday - 16 higher than the previous peak of 8493 prisoners on September 7.



The high number underlined the "urgent need" for more beds in the form of container cell blocks and double bunking, she said.



Labour law and order spokesman Clayton Cosgrove said the timing of the announcement, while the union of corrections officers was taking the Corrections Department to court over double bunking, was no coincidence.



"She's (Ms Collins') known about this before the election, she forecast that if all her election promises were fulfilled that it would require a doubling of the prison population."



Ms Collins said she released the information when it became available.



"It's a public safety issue and the public need to know," Ms Collins told Radio New Zealand.



Containers and double bunking will deal with 60 prisoners but will not solve the problem, she said.



Extra beds were needed for medium to high security prisoners.



Currently, there were enough cells but forecasts showed the Corrections Department would run out by February next year.



"This was predicted a couple of years ago when Phil Goff was the minister, unfortunately I found out when I came in to government that this had been predicted for some time and nothing had been done."



The prisons that had been built in that time were too small, Ms Collins said.



It was unlikely low-risk prisoners would be released to make space, she said.



"We don't have the right to just release people because we feel like it.



"It's not something that we want to be doing."