The Government's attempt at reforming our criminal justice system kicked off with Prime Minister Jacinda Ardern describing prisons as a "moral and fiscal failure".

It's the same phrase used by former Prime Minister Bill English in 2011 and came with Ardern telling the launch of the Government's criminal justice summit: "I don't see too much point in placing blame anywhere on this issue.

"No one set out to have a criminal justice system that has us ranking so highly when it comes to incarceration.

"I think we all realise that prisons are a moral and fiscal failure and that staying on a trajectory which would see us building a new prison every two to three years is even more so."

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About 400 people attended the launch tonight of the Government's criminal justice summit in Parliament, ahead of two days of meetings in Porirua to be attended by Government ministers, opposition MPs, national experts and international speakers.

Parliament's Banquet Hall saw judges and police rubbing elbows with gang members, Government ministers and former prison inmates.

To effect change, the Government needs some political consensus and there was an element of olive branch to Ardern's use of English's phrase and a rejection of blame over New Zealand's imprisonment rate.

English used the term as a platform to launch the National Party's social investment strategy, which aimed to be a cradle-to-the-grave data-driven exercise that targeted Government assistance to those who needed it most.

National leader Simon Bridges has said the Opposition will work with the Government on rehabilitation initiatives but warned of political backlash if reform meant changes to bail, parole or sentencing laws.

Ardern said she wanted New Zealand to be the land it already believed it was - "fair minded" and a place where "everyone is treated fairly".

"Sadly though, I think over the years the view we have of ourselves has been getting out of line with reality."

Ardern said New Zealand's ideal of itself had jarred in recent years against stories of homelessness and poverty.

"We have moved from strong communities that looked after one another to an individual approach to solving our problems."

She said many present would recognise "the impact of communities made up of individuals with little support, little connection and worst of all - the absence of hope".

Ardern said he interest in justice issues were sparked by hearing Principal Youth Court Judge Andrew Becroft - now Children's Commissioner - speak of common links between youth offenders.

They shared disengagement with education and work, shared family or gang violence along with alcohol and drug use linked to learning disabilities.

Those backgrounds led into prison statistics - 70 per cent had literacy difficulties, 62 per cent had mental health issues in the past year and 47 per cent had addiction problems.

Of those in prison, 40 per cent of men and 52 per cent of women had a lifetime diagnosis of PTSD from abuse and violence, with 53 per cent of women inside and 15 per cent of men having experienced sexual assault.

"So if we want to talk about an effective justice system, we shouldn't start with a discussion about prisons but a discussion about New Zealand."

The launch of the summit comes on a day the Government announced new funding for police which will see 1800 additional officers on the streets.

Critics have said the new police will see more arrests and higher prisoner numbers. Ardern emphasised "prevention" as a focus for police when speaking at the launch.

New Zealand's incarceration rate is second to the United States among comparable western countries.