Corrections Minister Kelvin Davis has done a swift backtrack saying he was aware of the latest prison population projections when the government made the Waikeria Prison decision.

Mr Davis had earlier told Morning Report he "hadn't seen the forecasts" when he last month announced the decision to build a 600-bed facility at Waikeria instead of a much bigger one.

Photo: RNZ / Dan Cook

New Zealand's prison population is being forecast to rise by more than 4000 over the next decade, according to a Ministry of Justice report. That would take the number of inmates to 14,400 by 2027.

The number of prisoners being held on remand - those awaiting trial or being held prior to sentencing - was predicted to almost double over that period to 5400. The report says that's because of an increase in the length of time people are spending waiting for their case to go to trial.

Mr Davis told Morning Report he saw the Justice Ministry report for the first time last night.

"I didn't personally [know about the forecast] when we made the [Waikeria] announcement," he said.

But several hours later Mr Davis released a statement which said he meant to say he hadn't read the public version of the report.

"The decision on the rebuild of Waikeria Prison was made by Cabinet based on the 2017 Justice Sector Projections," the statement said.

"My comments this morning were intended to convey that I simply had not read the public version of the report on the projections, which was published last week. Of course I've seen dozens of official reports based on the information in the projections."

National leader Simon Bridges said the "perplexing statements" showed Mr Davis wasn't across his portfolio.

"He's either not telling the truth or he's simply not up to the job - and sadly, I think the latter is probably the case," he said.

The government had to explain its plan in light of the fresh forecasts, he said.

"Either they have to build more prison beds to keep New Zealanders safe - or they have to soften up the bail, sentencing and parole laws."

Cabinet papers related to the Waikeria decision would be released within the next ten days, Mr Davis said.

Last month Mr Davis announced a 600-bed prison would replace the deteriorating jail at Waikeria. Plans to build a mega-prison, which would have been the largest in the country, had been scrapped in May.

"We made the decision to build the 500 beds at Waikeria and 100 mental health beds based on research," Mr Davis told Morning Report this morning.

He said the forecasts were on the basis of current policy and the new government was changing that.

"We're treating [the forecasts] as a warning and we're doing something about it," he said.

The government's research showed mega-prisons did not work, he said.

Mr Davis said since January, growth in the prison population was tracking below the forecast for the first time "in a very long time". There were now 10,500 people in prison, a fall of 300 since March.

National Party leader Simon Bridges told Morning Report the report stated it was finalised in November 2017 when the justice sector chief executives approved it "so what's been going on for the last several months?"

But Mr Davis, in his statement, said Mr Bridges was wrong.

"Ministers have been working off the 2017 projections through all decisions made in relation to Waikeria Prison. These formed the basis of the paper I took to Cabinet in June."

Mr Davis has previously said the government aimed to reduce the prison population by 30 percent over 15 years, to around 7000.