An Auckland lawyer says there is likely to be many more prisoners unlawfully detained by the Department of Corrections than the 17 awarded compensation in the past six years.
Since 2006, 17 people have received nearly $200,000 for unlawful detention after serving a combined total of 438 extra days.
People who received compensation were kept behind bars for between three and 92 days after their correct release dates.
Corrections says all the cases were due to administrative errors, which can arise when judges or court staff record the wrong dates or when time served is miscalculated by prison staff.
The biggest payment was $27,000 to an inmate who was unlawfully held for nearly three months at Hawke's Bay regional prison in 2006.
The department says it costs about $250 per day to house a prisoner.
Barrister Jeremy Sutton represented seven of those affected and says often people do not bother to seek compensation for unlawful detention as the legal process can take up to two years.
Mr Sutton says unlawful detention often occurs because time spent on remand is not factored into a prisoner's sentence.
Another deterrent is the Prisoner's and Victims' Claims Act, which allows the Government to direct any compensation to unpaid fines, legal aid bills or to victims, he says.
Corrections Minister Anne Tolley says she has told her department to get its records straight.