A pilot of a special court that diverts offenders with alcohol and drug dependency from prison into treatment has been extended.

Justice Minister Amy Adams said initial analysis showed the Alcohol and Other Drug Treatment Court reduced the likelihood of reoffending by about 15 per cent in the short-term, compared to similar offenders who went through the usual court process.

The radical court pilot began in 2012 in the Waitakere and Auckland District Courts. Funding was due to stop in June, but today it was confirmed the pilots would be extended for another three years.

"The early signs are incredibly promising. Indications are that just those pilots already have probably seen about 61 prison beds saved," said Adams.

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After Adams outlined the court's success to Parliament's justice and electoral committee today, Green Party MP David Clendon asked why the courts could not be rolled out across the country.

"There is international experience that they actually work. I just wonder why we need another three years of trying," Clendon said. "Most of the regions have high rates of recidivism with drunk driving and these courts do seem to break that cycle in a way that prison sentences, fines, loss of licences don't.

"Another three years - realistically if then we get a go ahead, and take another couple years to build infrastructure, it will be a decade from the first kick-off."

Adams said she asked her officials the same questions, and the 42 people who had graduated from the programme so far was too small a sample size to make such a call.

"All the statisticians and evaluating panels tell me it's not large enough yet to do a proper evaluation."

Offenders can't choose to appear before the Alcohol and Other Drug Treatment Court. Potential participants are identified when appearing at the Auckland or Waitakere District Courts.

The process is for people who have committed less serious crimes and who admit their guilt.

Once people have undergone treatment - a process that can take up to 18 months - they are still sentenced for their crime. Odyssey House is the lead provider for assessment and treatment services, and works with Higher Ground and the Salvation Army.