Bureaucratic bungling has seen taxpayers fork out tens of thousands of dollars in compo payments each year to criminals kept in jail too long.
A Waikato Times investigation has revealed that in the past five years $127,800 was paid to prisoners wrongfully detained beyond their statutory release date.
Reasons for the lapses range from errors in sentence calculations to incorrect information being supplied to prison officials.
Corrections staff say they take the issue of sentence miscalculation seriously but one former prison boss said wrongful detentions were inevitable due to inmates serving multiple sentences.
Last year, $27,800 was paid to three prisoners wrongfully detained, with one Christchurch prisoner serving 187 days beyond his statutory release date.
In that case, a probation officer provided the Parole Board with incorrect information.
Corrections Services deputy national commissioner Maria McDonald said sentence calculation was a complex and "technically difficult task" performed by prison staff and based on information from the Justice Ministry.
The department took errors seriously and was working with the courts and police to improve the quality of information between agencies.
"I appreciate that any oversight is disappointing, especially when that oversight results in the Crown incurring a financial liability," Ms McDonald said.
"During the last five years we held over 108,000 prisoners in custody. Only a small percentage of the total prisoners held in custody have been wrongfully detained; this serves to illustrate that these oversights are very rare in nature."
Rethinking Crime and Punishment executive director Phil McCarthy said the public was right to be concerned by wrongful detentions but such incidents were inevitable.
Mr McCarthy, who was Public Prisons Service general manager from 1995 to 2006, said there was always a "steady trickle" of such cases historically.
"The public will look at this and say ‘how can this happen?'. On the face of it it seems absurd that people would be kept in prison too long but it can be extremely complex, especially when you've got someone on multiple sentences. Some of the calculations you need a degree to work out." It was essential compensation was paid to prisoners wrongfully detained, he said.
But Sensible Sentencing Trust spokesperson Ruth Money objected to prisoners being paid compensation when the vast majority did not serve their full sentence. "In many cases, prisoners are released after serving a third of their sentence. The fact they could be paid money simply because they're kept inside a few days or weeks too long doesn't seem appropriate.
"I don't want someone who steals a car doing 100 extra days but if it's a serious violent criminal or a repeat offender, then I kind of go hmmm.
"From a victim's point of view there are plenty of injustices that never carry any compensation."
Ms McDonald said the department took a pragmatic approach to compensation.
In most cases the potential cost of litigation was greater than the sum claimed by prisoners.
Any prisoner compensation is subject to the Prisoners' and Victims' Claims Act 2005. The act requires any compensation be paid to the Justice Ministry, which must deduct any victim claims and unpaid reparations before payment is made to the prisoner.
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