Both double-bunking and container cells will be needed to avoid a prison bed crisis due to peak early next year, Corrections Minister Judith Collins says.

Prefabricated or container cells alone would not solve the crisis, which will see all beds filled by February, Ms Collins said today.

She said the issue would be addressed in three stages, with container or modular cells and double-bunking a short-term solution, prison extensions a medium term one and a new prison, likely to be at Wiri in south Auckland, a long-term measure.

Forecasts suggested about 5000 extra beds were likely to be needed by 2018.

Ms Collins said the double-bunking and use of shipping containers were separate issues.

"In addition to that we have already made a decision about having private management of prisons. The law is drafted and is in the select committee at the moment and we have every intention of going through with that."

The double-bunking issue needed agreement between unions and prison management.

"We have already made a decision that there will be double bunking, but we have to negotiate terms with the unions because under their collective contract that's not something we can do unilaterally," she said.

Corrections said it had met continually with the Corrections Association and Public Services Association this year about double-bunking.

The department would not comment in detail while consultation was ongoing so as to "give these discussions the best chance of success".

The container or modular issue was still being discussed, but Ms Collins said she had asked the Corrections Department today to consider it urgently.

The shipping container option has come under fire from human rights campaigners.

Rethinking Crime and Punishment director Kim Workman and Auckland QC Peter Williams have described the idea of putting prisoners in containers as inhumane and degrading.

Ms Collins said the containers could be set up for use and connected to electricity and plumbing services for between $53,000 and $63,000 each.

She has suggested that prisoners could help set them up.

- NZPA