Corrections Minister Kelvin Davis says New Zealand "averted a catastrophe" by voting out the National-led Government.
Had they been in power another three years, he told Newshub Nation on Saturday the prison population would now be "close to 12,000".
It's presently just under 10,000, after falling 7 percent in the past 12 months - still above the present safe capacity of 9161, according to Corrections.
"We've reduced the prison population by 7 percent in the last year, and it gives us greater flexibility to do things, but we have to do things differently," Davis said. "The cost to the country is greater if we don't."
This week Davis announced a new $98 million plan to help prisoners make their current stay behind bars, their last.
"It's aimed at all people, but it's going to be based on kaupapa Māori - Māori values, Māori history. If other people want to be involved in that, they're more than welcome."
Māori currently make up 15 percent of the population, but half of all male prisoners. For Māori women, it's worse - they make up 60 percent. If Māori imprisonment rates matched those of other ethnicities, the prison population would drop 44 percent - far in advance of the Government's 30 percent target over the next 15 years.
"I came into politics to make a difference for our people, our Māori people, and the incarceration rate of our people is something that cannot be tolerated anymore," said Davis, whose new plan brings together Corrections, Whanau Ora and the Ministry of Social Development.
"The Finance Minister has asked us to pool our resources and our skills together so that we can give this pathway for Māori prisoners from the time they enter the prison system to well after they've left."
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In the final two years of the National-led Government, the prison population spiked - going from 8834 in 2015 to 10,470 in 2017. Corrections projected it would hit 12,000 by 2022, but the latest figures show it's been trending downwards since April last year.
Corrections says overcrowding makes rehabilitation programmes - which already cost the country $200 million a year - less effective.
"I didn't expect us to have such success so soon," said Davis, who criticised National's policy to "every three or four years, build a billion-dollar American-style mega-prison".
"We've averted a catastrophe."
But there's a long way to go - particularly for Māori.
"What we've got now is a situation Māoridom cannot sustain, having our young people locked up. That's why we're targeting the hard-to-reach prisoners - people, the cohort 30 years and younger, they're the people most likely to go on and reoffend.
"What we want to do is break the cycle of intergenerational imprisonment. I was at a prison last year where there was a man, his father and his grandfather in prison. We've got to stop that. Māoridom depends on this succeeding."
The new programme will start in prisons at Ngawha and Hawke's Bay, and Davis hopes it'll expand to other facilities - including those for women.
"This isn't about tinkering around the fringes and the edges - this is about systemic change."