The latest advice the Government has received on the prison population is that over the next 10 years it will increase by 4100 places, advice it had when it made its decision to scale back the Waikeria expansion.

The projection is more than 2000 extra prisoners on the previous year's, prompting National leader Simon Bridges to describe the decision to scale back Waikeria plans as "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.

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The National Government had used the Ministry of Justice's 2016 annual projections in making its decision to expand Waikeria to 1500.

"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 latest projection was made in October last year, the same month the Labour-New Zealand First Coalition was formed. They do not factor in policy announcements made since then.

It includes projections for people held on remand and those sentence.

The 10-year projection is that by 2027, there will be 14,400 prisoners.

That is a not only 4100 more than the 2017 muster of 10,300, but a big jump on the 10-year projection, in 2016.

In 2016 the 10-year projection was for an increase of 2350 extra prisoners, to 12,200 in 2016.

The latest projection is well in excess of the 2015 projection which estimated there would an increase of only 950 in the prison population between 2015 and 2025.

The Government announced its plans for Waikeria in June, which include a 500-bed high security and a 100-bed mental health facility for prisoners – to be completed by 2022.

National had been planning a 1500-bed rebuild, with the capacity to hold 2000 prisoners.

In making the announcement, Corrections Minister Kelvin 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 in providing 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 to five prisons: 244 to Rolleston Prison; 122 to Tongariro Prison; 244 to Rimutaka; 122 to Christchurch Women's Prison; and 244 to Christchurch Men's Prison.

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

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.