UPDATED 6.09pm The Government has announced plans for a growing prisoner population including double-bunking for an extra 80 beds at Ngawha in Northland, a new building at Mt Eden to take 245 extra prisoners, and possibly a new 1500-bed prison on the current Waikeria Prison site in Waikato.

LISTEN ABOVE: Jane Maltby of Howard League for Penal Reform spoke to Larry Williams

Corrections Minister Judith Collins said prison population growth required a further 1800 places.

Work was already underway to add 341 prison places through double-bunking and converting facilities to accommodate more beds.

But another 1800 places were needed which would cost $1 billion.

Collins said a business case for a new Waikeria prison would be considered next week.

It would be operated by Corrections but built and maintained by a public-private partnership which was the same model being used in a new maximum security facility in the grounds of Auckland Prison, at Paremoremo.

She said the proposed double bunking at Northland would be in place by early 2017; the new accommodation block at Mt Eden would be in place by late 2019; and if approved the new Waikeria Prison would get its first prisoners in 2020.

But Green MP David Clendon said the Waikeria prison is not convenient to anywhere and is an extremely difficult place for inmates' families to get to.

He said he'd invite the Minister of Corrections to get to Waikeria prison using public transport, as if she was a low income spouse of an inmate at that prison.

Mr Clendon says it's simply the wrong approach.

The Government's been taken to task in Question Time over its planned prison expansion, with Labour pointing out that in 2011 Finance Minister Bill English promised to build no more prisons.

Mr English admits he turned out to be wrong.

But he said they had no choice, as the courts are sentencing more people, and they need somewhere to lock them up.

Collins said that despite "significant progress' in reducing crime, the number of prisoners had increased faster than expected.

"This is because the proportion of offenders charged with serious crimes has risen, meaning more people are being remanded in custody and serving more of their sentences in prison.

"We have to respond through new investment or we will create unacceptable safety risks for staff, prisoners and the public, and be less effective at rehabilitating prisoners."

She said the next stage of the prions programme would also target the drivers of crime, drug and alcohol abuse and domestic violence.

Labour's Andrew Little said it's an indictment on failed policies that the Government is now having to spend a billion dollars on providing another 1800 prison beds.

Mr Little said putting in place the right support and social structures, including a well-resourced police force, doesn't mean you have more prisoners, you should have fewer.

He said the presence of more police in our communities would actually deter crime.

Corrections was proposing an increase to the delivery of rehabilitation programmes, including drug treatment units, reintegration programmes, education and training programmes, and special treatment units to help address violent and sexual offending.