Six overseas Tongans have been deported to Tonga or have voluntarily left New Zealand after their criminal records were wrongly wiped-off by Tonga Police under a former practice that remains under police investigation in Nuku'alofa.

Immigration New Zealand General Manager Peter Elms said they had discovered 12 unlawful individuals, including the six who had returned to Tonga, including the three who were deported to Tonga in March.

Another seven hold temporary entry class visas, and one of these has been served with a deportation liability notice.

He told Matangi Tonga that the investigation continued on all cases of people involved, to determine those who may be liable for deportation.

"But further information is still needed in the case of some individuals, and it is important to stress that every case is considered on its merits before decisions are made," he said.

"Immigration alerts have been placed on all those names on the list where there is sufficient certainty about the individuals identity, in case they try apply for a further visa or enter the country."

Immigration NZ had initially confirmed 52 Tongan nationals with wrongly wiped convictions, who are believed to be in New Zealand. They were on the list of the 172 names provided by the Tongan authorities.