National leader Simon Bridges has accused the Government of trying to hide a report claiming the prison population is set to skyrocket.
The Ministry of Justice report, presented in October 2017 - the same month the Labour-NZ First coalition formed - projects New Zealand will have an extra 4100 people behind bars by 2027. The previous year's projection was for only 2350 more.
The Government has a goal of reducing the prison population by 30 percent over the next 15 years, and scaled back a planned mega-prison build at Waikeria from 1500 beds to 600.
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"We already knew there was a prison shortage of beds - now we know though actually the forecast prison population is set to increase by a couple of thousand more than they thought," Mr Bridges told The AM Show on Monday morning.
"So look, it's pretty simple, isn't it? Ultimately New Zealanders outside - because they're going to have to soften up the bail, the sentencing, the parole laws - will be less safe."
The 2016 projection for 12,200 prisoners by 2027 led the then-National Government to put in plans to expand Waikeria. The 2017 projection for 14,400 prisoners came before the Government decided to abandon what it called the "American-style" mega-prison.
Prime Minister Jacinda Ardern said in June bigger prisons don't "necessarily make us any safer".
"I want the people who need to be in prison to be in prison, and the people who should, or perhaps could, benefit from early intervention and rehabilitation to have that. All we want ultimately is an effective justice system, and that's what this debate is about."
The Government has already admitted double-bunking looks here to stay, and some prisoners might be required to sleep on mattresses on the floor. Corrections Minister Kelvin Davis said in June that was a "budgetary" decision, rather than ideological.
"We don't want to double-bunk. We don't want to build prisons in the first place, but, you know, that's where we've landed."
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Mr Bridges stopped short of saying the Government had lied about the Ministry of Justice's revised projection, instead calling them "politically dishonest".
"This is a Ministry of Justice report, it's signed off by the chief executives in the justice sector and it gives the authorative view on what's happening with the prison population. It's really irresponsible because what it means ultimately is New Zealanders on the streets - but also inside the prisons, the prison guards and the prisoners - are less safe."
The 2017 projections, having been issued the same month the Government was formed, don't take into account decisions made by the Government since then.