Police have spent nearly $7 million in the past three years relocating witnesses in high-profile criminal cases and giving them new identities.
Witness protection programme figures were released to the Herald on Sunday under the Official Information Act, but police would not say how many were under protection.
It is believed most of the money is spent on new passports, relocation costs and living allowances.
The programme is also used for criminals giving evidence against their own.
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Officer in charge, Inspector Phil Jones, said revealing any details about them or their whereabouts could put the witnesses in danger. He said criminal groups could also start pressuring friends and family to find them.
"Police give a commitment to these witnesses that we will do everything possible to protect them from future danger, and I am firmly of the opinion the release of the data requested will jeopardise their safety and the relationship based on trust with the police," Jones said.
The Police Association said it was important details about the programme were not revealed. "It's a valuable service and without programmes like this, it would make it hard for people to give evidence in some high-profile court cases," said vice-president Luke Shadbolt.
"But I won't make any comment on the specifics. It comes down to common sense. These people are in witness protection and the less information we give out about it, the better." In 2007, a ministerial inquiry was launched after convicted criminal Jonathan Barclay, who was in the witness protection programme, killed a woman near Nelson in a 2006 car crash.
A month earlier, he could have been jailed for a repeat driving offence but was treated as a first-time offender because of his new identity from the witness protection programme. He has since been jailed for his part in another crash.
Meanwhile, police have spent $700,000 in the past three years paying informants.
Inspector Paul Berry, acting national crime manager, said police could not reveal the type of cases information was paid for or where people lived because it would "prejudice the maintenance of the law, including the prevention, investigation and detection because ... it would indicate to criminals the potential levels of covert police activity in this area".