Prisoners released too soon from prison went on to commit more crime.

Fifteen inmates were mistakenly released from jail in the past three years, figures from Corrections' show.

The inmates were released from prisons around the country, but Northland Region and Auckland's Mt Eden prisons had the highest error rate, with three inmates from each let out early.

The most serious offending by inmates released in error was a man facing an aggravated robbery charge and incarcerated at Auckland South.

A Manawatu Prison inmate mistakenly released was either charged with assaulting a woman or was serving time for the offence. It was not clear whether those behind bars were on remand or serving a sentence.


Others mistakenly released early were either facing charges or serving sentences for driving, drug and dishonesty offending.

The Weekend Herald revealed in June recividist Trade Me fraudster Joshua Calthorpe was mistakenly released from Serco-run Auckland South prison at Wiri. He finished a sentence but should have been transferred to a remand cell because of other charges.

Instead he was free for two weeks before returned to custody.

Calthorpe was later sentenced to almost six years in jail for aggravated robbery - which occurred when he held a knife against a young tourist and threatened to stab her - blackmail and obtaining by deception.

SecureFuture, the consortium that owns the Serco-run prison at Wiri, was fined more than $26,000 for the wrongful release and received 26 service failure points.

A spokeswoman for Serco, the private prison operator that last year lost its contract to run Mt Eden prison, said a review was done to prevent a repeat. Corrections' national commissioner Jeremy Lightfoot said the errors were "disappointing" but sentence calculation was a "complex and technically difficult" task based on information from the Ministry of Justice.

The department was working on improvements and measures taken include improved data sharing between the department, courts and police, checking with courts and a support service for those calculating release dates.

"However, the number of wrongful releases needs to be viewed in context ... in any one year there can be upwards of 15,000 prisoners received into custody."

Corrections is not penalised for wrongful release from prisons it manages, but Serco was fined $25,000 after a mistaken release in 2014. Police were called as soon as it was discovered a prisoner had been wrongfully released.

Lightfoot refused to say how long each prisoner was at large, saying that would likely prejudice maintenance of the law and the right to a fair trial.

New Zealand First Corrections' spokesman Mahesh Bindra, a Corrections' officer for a decade to 2014, said staff used a calculator and an Excel sheet to work out when inmates were due for release.

Training was inadequate and the department did not invest in software to make the process fail safe.

"Every wrongful release is a ticking time bomb in the community."

Lightfoot said information provided by the courts had been entered into the Corrections Integrated Offender Management System since 1998 and release dates calculated by the system were manually verified. by trained staff.

Labour Corrections' spokesman Kelvin Davis said the errors were "another example of Government agencies not talking to each other".