Former prisoners are seeking legal advice over possible unlawful detention following a Supreme Court ruling.
The court ruled last week that the Department of Corrections had held some prisoners in jail for longer than they should have been.
The Supreme Court ruled the department had failed to factor in time spent in custody on remand when calculating Michael Marino's release date. The department had been using the same formula for setting release dates since at least 2003.
On Friday, Corrections released 21 prisoners identified as being immediately affected by the court decision, and deputy national commissioner Rachel Leota said about 500 people currently in prison had release dates that would "likely" need to be changed.
RNZ News understands that, in Wellington alone, at least 12 former prisoners have now sought legal advice over possible unlawful detention.
When the decision was released, Michael Marino's lawyer Douglas Ewen said thousands of former prisoners might seek redress.
However Corrections Minister Judith Collins said the department would "rigorously" defend any compensation claims made by prisoners for being jailed too long.
She said the essence of the Supreme Court decision was that it overturned the decisions of other courts in the last 14 years that had interpreted the law differently.