Prisoners could be forced to build their own jail cells from shipping containers to cope with a "dangerously high" prison population that is expected to spill over in six months.

The Key government has asked Corrections to produce options to cope with the burgeoning prison muster, which has increased by 700 so far this year.

It is the latest of the government's hardline "tough on crime" measures, which have included removing parole eligibility for the worst repeat violent offenders, allowing the cars of illegal street racers to be crushed and the seizure of the assets and profits of gangs. Police have received powers to issue on-the-spot protection orders in family violence cases, and prison release conditions for serious child-sex offenders have been tightened.

Corrections Minister Judith Collins said the issue of housing prisoners was expected to reach crisis point in the new year. Double-bunking the standard practice of putting two prisoners into a single cell was not sustainable and the economic downturn meant the government could not afford to build new prisons in the short term.

Using prisoners to build their own cells was "a great idea" and "a lot better than being locked up all day in a cell", Collins said.

"We are getting dangerously high in our capacity. We will not have the capacity by the beginning of next year to house all the prisoners that we will have."

Corrections has yet to respond to Collins on the proposal for shipping-container cells, which she said would "be spartan but humane and clean. We are looking at whether we could make good use out of prisoner work teams to help build these, and obviously things like landscaping. We're quite keen to have prisoners learning useful construction skills and helping to build their own environment. Prisoners need to learn construction skills so they can earn their keep and, frankly, it's a lot better than being locked up all day in a cell."

Kim Workman, the director of alternative justice thinktank Rethinking Crime and Punishment, said the idea of housing prisoners in shipping containers was inhumane, and forcing prisoners to fit them out was asking for serious trouble.

"A container is a container and to have prisoners living in containers... this is stuff they were talking about in Australia 10-15 years ago and the department decided then that what was happening was inhumane.

"This is likely to create a considerable backlash. I think they are going one step too far with the idea of housing prisoners in containers. I think it's a major breach of human rights and a contravention of the United Nations minimum rules for accommodating prisoners."

Workman did not have a problem with prisoners working, "but this is a little bit like asking a person who's been sentenced to hanging to build their own gallows. You can imagine how they would feel, building these atrocities and then being expected to live in them you might get a very negative reaction from prisoners.

"I think she [the minister] is on the verge of creating a situation where there will be major riots and people will die."

But Collins said the shipping containers would provide a better standard of housing than some of the country's older prisons such as Auckland's Mt Eden and Wellington's Mt Crawford.

The average prison population in 1996-97 was about 5000. Today it is about 8500 and is continuing to rise. New Zealand is said to have the second highest rate of imprisonment among western countries, after the US.

Collins said the issue was not so much overcrowding, but having too many prisoners concentrated in certain areas of the country. It was expensive and unsafe to constantly transport prisoners and it was also wrong that some were moved far from their families.

The shipping containers would give prison authorities more flexibility in managing the inmate population. Auckland, Rimutaka and Hastings were the immediate pressure points, and the intention would be to put the containers on land next to existing prisons, effectively increasing the bed-count.

"The previous government did not actually build enough space and bunks basically, and we've been left with this to deal with," Collins said.

The cost of building accommodation to the standard of the new Spring Hill prison in Meremere, south of Auckland, works out at about $643,000 per bed. Using shipping containers, the cost is an estimated $380,000 per bed.

Opposition corrections spokesman Clayton Cosgrove accused Collins of playing popular politics and said the proposal would create more tension for prison officers. "It's a nice little slogan to whip out you know, let's have them in shipping containers but there's no detail."

Meanwhile, the government's election promise to scrap parole for prisoners who refuse to take part in jail work programmes has hit a legal snag. Collins says she is pushing ahead with the plan but admits there are human rights concerns.

National promised to amend the Parole Act to ensure prisoners who could work but refused were not eligible for parole, and Collins believes the legal issues can be overcome.

The Corrections Act allows prison officers to direct inmates to work, but only in chores such as laundry, cleaning and prison maintenance. Work that requires labour market skills such as building or forestry is voluntary and prisoners are paid less than the minimum wage. Only low-risk prisoners can work outside prisons.

Collins said the prisoners who might benefit the most from learning work skills often chose not to help themselves. "If you are ever going to break the life of crime, it is through work, education and spiritual values."