Thousands of prisoners' jail time could have been wrongly calculated, the Minister of Corrections says, after an earlier estimate of 500 serving prisoners.
Following a Supreme Court decision released on Thursday, the Department of Corrections said about 500 of those currently in prison had release dates that would "likely" need to be changed.
However, Minister of Corrections Judith Collins told Checkpoint with John Campbell that more cases would come to light as the department carried out a review going back to 2002.
When asked whether the numbers would be in the thousands, she replied, "I think that there might be, over the years, because you're talking about 14 years' worth."
Corrections has about 9700 prisoners right now and, of that group, about 500 would be affected, she said.
"Corrections has said in some cases it will be people who have served some days more, some weeks more and there may even be some with some months more."
In its judgment, the Supreme Court ruled a violent offender, Michael Marino, spent about four months longer in jail than he should have because the department had failed to accurately factor in the time he had spent on remand.
Corrections has been recalculating release dates and some prisoners were due to be freed as early as today as a result.
Marino's lawyer, Douglas Ewen, said earlier that sentencing laws had been confusing for years and thousands of prisoners could be affected.
He said that could cost the department millions if it had to pay compensation.
But Ms Collins has questioned that.
She said earlier today the essence of the judgment was that it overturned the decisions of other courts in the past 14 years, which had interpreted the law differently.
The court's new interpretation did not align with the way her department - or the Court of Appeal - had understood it to operate, she said.
"It's clearly not what the Court of Appeal believes the law should have been interpreted as, and we now have this new interpretation that the government and Parliament will need to consider, as whether or not that is the right place for the law to be."
She told Checkpoint the department would "rigorously" defend any compensation claims made by prisoners.
"And they would be defended on this basis. That Corrections in 2002 was given legislation by Parliament, the Parole Act, and they interpreted the act a certain way", Mrs Collins said.
"That has apparently been upheld over four times by the Court of Appeal, and if Corrections had, in fact, interpreted the act other than what the Court of Appeal said they should, then they would have been outside the law."
Humans right lawyer Andrew Butler, however, said he believed the department could not rely on a claim it was acting in good faith.
Even if the department had been acting in accordance with the law as defined in previous cases, that would not preclude compensation claims, he said.
"To use the technical legal term, the tort is complete upon the court reaching the conclusion that the person who was detaining another person had no legal authority to do so.
"And effectively what the Supreme Court has said is that Corrections had no legal authority to detain these people for as long as Corrections had been detaining them."
In Thursday's judgment, Justice William Young suggested the Parole Act might need to be reconsidered by Parliament.
Ms Collins said, while she was not a fan of retrospective legislation, she could not rule out the possibility that the government would ask Parliament to change the law.
"There is a court higher than the Supreme Court and that is Parliament," Mrs Collins said.