Prison inmates could soon find themselves forced to share a room, as the Corrections Department talks with staff about putting two beds into every standard prison cell.

Corrections Association president Beven Hanlon said the move would add an extra 950 beds to the jail system, covering expected growth in prisoner numbers in the next 18 months.

He said the move - known as double-bunking - would be on top of new prisons, and would be permanent.

The extra beds would be added in the four new jails opened in the past four years.

Putting two prisoners in each cell has previously been rejected by the Corrections Department as it believed the standard 6.5sq m cell was not suitable for two people.

Such close-quarter living had the potential to cause "issues of inmate compatibility and tensions", the department has said.

The proposal has horrified criminologist Greg Newbold, who himself spent time in prison.

He said the plan would be "disastrous", and lead to increased violence towards inmates and staff.

"When you're locked up with someone you can't get away from [or] with someone you may detest, people will explode in that type of environment," Dr Newbold said.

"It's the worst thing they can possibly do. It leads to homosexual rape and bullying in cells, and dehumanises the inmate.

"Any inmate doing a long term knows the hour you look forward to is lock-up time - time on your own without hindrance from another inmate."

Dr Newbold has researched prisons in the US and Australia, where cell-sharing is common.

In Louisiana, he observed five-bunk stacking, as well as dormitory-style rooms with 100 to 200 people inside.

As well as rapes, stabbings were also common in such environments, he said. "It is a cheap and nasty solution. The beginning of the decline of Corrections in this country."

Mr Hanlon said Corrections had told the union three weeks ago that it needed an extra 950 beds through double-bunking all cells in the four new jails, apart from at-risk cells and drug and alcohol treatment units.

"We are very close to full," he said.

Mr Hanlon is visiting prisons to gauge staff reaction to the proposal. He is due at Auckland Prison at Paremoremo today.