Former prisoners are calling their counsel to check if they were held in jail for too long after a Supreme Court ruling raised the prospect of compensation for hundreds.
Hamilton lawyer Roger Laybourn said he had received calls from former prisoners who thought they could be affected.
"I wouldn't be surprised if this is happening up and down the country - that lawyers like myself will have ex-clients ringing and saying, 'I've long had a beef with the prison about my release date'."
The lawyer for the man who appealed to the Supreme Court and has been found to have spent four-and-a-half months too long in jail says they will seek compensation.
Corrections Minister Judith Collins has indicated the Government will now change the law.
Offenders could get about $6600 for each month wrongly spent behind bars, based on a past case.
But after a briefing from Corrections today Collins said the chances of taxpayer money being paid out were "remote".
"I think if I was any of these offenders I would not be going out to buy a new car based on what I think I might get," she said.
The incorrect interpretation of the law dates back the 2002 Parole Act - meaning hundreds of people could have spent too long inside.
Corrections will release 21 prisoners this week, and says about 500 current inmates could have their jail terms slashed.
The Supreme Court ruling could create "very perverse" outcomes, Collins said - including that prisoners could offend while in jail on remand without any extra time being added to a sentence.
"That is obviously something we need to look at. But it's very early to talk about legislation. There may well be a need to revisit the Parole Act."
Collins strongly defended Corrections, saying the department had done nothing wrong and had acted in accordance with how the Court of Appeal ruled the law should be interpreted on four occasions.
Time held in detention before a person is sentenced is treated by the law as time already served when release dates and parole are determined.
The Supreme Court has ruled that time in detention starts from the point of arrest - even if other charges are later laid.
Corrections has instead been working it out on a charge-by-charge basis.
This was done in the case of Michael Marino, who took the appeal to the Supreme Court. Marino was taken into custody in February last year on family violence charges.
In March and June further charges were laid of attempting to pervert the course of justice as a result of telephone calls he made from prison.
Marino pleaded guilty to all charges and was jailed for 22 months, less any time already held in detention.
Corrections worked out he would get out of jail in May this year - calculating time in detention from when the second perverting the course of justice charge was laid.
That was June 19 last year - meaning he didn't get credit for the period from February to June when he was held on family violence charges.
Marino's appeals were unsuccessful in the High Court and Court of Appeal, but the Supreme Court granted him leave to appeal.
That mattered to another offender, Edward Thomas Booth who was also affected by Corrections' approach to calculating detention.
Marino is no longer in custody, but the ruling means Booth no longer needs to pursue his appeal and will have his eligibility for parole brought forward by about 10 months.
His lawyer Andrew Bailey said that would probably be some time next year. His client was "very happy" with the ruling.
Collins said if the Parole Act was changed it could be done so in such a way as to rule out compensation.
However, she said under the Prisoners' and Victims' Claims Act any compensation would likely be considered a windfall and be paid to victims, and the Limitation Act could also limit claims.
Corrections had also been acting in accordance with the law as determined by the Court of Appeal, Collins said.
Green Party Corrections spokesman David Clendon said the department had clearly got it wrong and was in urgent need of an overhaul.