Corrections Minister Kelvin Davis has clarified confusion over what advice he had seen before the decision to scale back the Waikeria prison expansion planned by the previous Government.

He said on Radio New Zealand (RNZ) this morning he had not seen the Ministry of Justice report from last October forecasting an increase in the prison population by 4100 over the next decade. He saw the report for the first time last night.

National leader Simon Bridges described that as "staggeringly incompetent".

But Davis has since said ministers had been "working off the 2017 projections through all decisions made in relation to Waikeria Prison".

Davis said he had meant to convey in his radio interview that he had not read the public version of the report.

"Of course I've seen dozens of official reports based on the information in the projections."

National had planned to rebuild Waikeria to a 1500-person prison and capacity to go to 2000.

Davis announced last month that the expansion would be scaled back to a 500-bed prison and a 100-bed mental health facility.

Long-term projections for the prison population are prepared annually by the Ministry of Justice. The 2017 report was completed in October and issued only in the past few days.

The projected rise of 4100 prisoners is well above the 2016 forecast, which was a rise of 2350 prisoners to 12,200.

The latest projection is well in excess of the 2015 projection, which estimated an increase of only 950 in the prison population by 2025.

Bridges said the latest projections made the Waikeria decision more "egregious".

"The Government has sat on a report for over six months showing that the prison population is forecast to reach 14,400, up from the previous forecast of 12,200, yet in that time made the decision to reduce the planned number of beds at Waikeria Prison from 1500 to just 600," Bridges said.

"Reducing the number of new beds at Waikeria Prison by almost 1000 shows a disconnect with reality and disregard for the safety of New Zealanders," he said.

"New Zealand will be hundreds, perhaps thousands, of prison beds short, which means the Government won't be able to put away all the serious offenders it needs to."

The Government announced its plans for Waikeria in June with a completion date of 2022.

Davis said the easy thing to do would have been to build an American style mega-prison.

"It's just not the right thing to do."

He said smaller prisoner were more effective for real rehabilitation.

As well as providing 600 new beds at Waikeria, he would be adding 400 extra beds across the prison network.

In the meantime, by the end of next year 976 rapid-build temporary structures would be added: 244 at Rolleston Prison; 122 at Tongariro Prison; 244 at Rimutaka; 122 at Christchurch Women's Prison; and 244 at Christchurch Men's Prison.

The Government has also budgeted $57.6 million over four years to house and support 300 people in response to many prisoners being ready for release but having no suitable housing.

The Government has set a target to reduce the prison population by 30 per cent over 15 years.

Justice Minister Andrew Little will be holding a summit in August, which is expected to contribute to a plan to cut the prison population.