New Zealand's prisons are increasingly making inmates share rooms and the system is "stretched", but jails won't be running out of beds, the head of Corrections says.

Chief executive Ray Smith on Thursday told parliament's justice select committee the past three years had seen the fastest period of growth in the department's history with the male prison population growing 18 per cent and the number of women up a staggering 50 per cent.

"The numbers have been much larger than anyone could have reasonably anticipated," he said.

But despite reports last month showing only about 300 beds had been left free in prisons, Mr Smith was adamant jails wouldn't be running out of space.

"We've been certainly stretched, and at times we've been at our limits, particularly with our female population," he said.

"We're not going to get to that point, I'm pretty confident of that."

He said double-bunking - where two prisoners are kept in one room - was becoming increasingly common but still well below levels in parts of Australia and Britain.

"Double-bunking was sitting at about 22-23 per cent of the prison estate. It's now probably closer to about 40 per cent of the prison estate," he said.

"Currently we've had to put people into smaller cells than we ideally would for double-bunking because it was the only option we had."

But he said double-bunking was not fundamentally a problem as long as other facilities at the prison could keep up.

New Zealand laws also prevented prisons from running over capacity - simply because restrictions legally limited how many people could be in the building, he said.
"We don't overcrowd our prisons in New Zealand. I can't overcrowd the prisons."

Asked about the rise in female crime, he said violent crime and drugs were key drivers of figures.

Almost no prisons had not had some kind of extension or improvement in the past three years.

The government has set a goal of reducing the prison population by 30 per cent over the next 15 years.

Corrections Minister Kelvin Davis on Thursday said while that process would be broken into phases, it would take the full 15 years to achieve.

He is expected to soon decide whether to proceed with a 3000-bed extension to Waikeria Prison in Waikato planned by the previous government.