Ronny Knight says the current facility is built to hold about 20 people, but there are currently 200 inside, which he blames on a booming population and illegal logging.
He says nobody from Australia's asylum seeker detention centre on the island is kept in this facility.
Mr Knight told Radio New Zealand International, a new prison is planned for the next couple of years, but wider reforms are needed as many of the inmates are locked up for minor offences.
"Maybe we should look more at trying to get people on probation.
"Some of these people are in jail for things they wouldn't be jailed for in other countries, minimal things that could be put out on probation or they could be given community service.
"So we're looking at trying more prevention in looking for a cure here," Mr. Knight said.