Police spent $640,000 rescuing baby Kahu from the kidnapper who demanded a $3 million ransom for her return.

Kahurautete Durie was held for eight days in April before armed police spirited her away then arrested the kidnapper after lying in wait outside his King Country hideaway for three days.

Figures obtained by the Herald under the Official Information Act show $519,428 was spent on the wages of those working on the operation. An additional $121,383 was spent on "operating expenses".

Although the Government had offered police the $3 million demanded from baby Kahu's high-profile parents, High Court judge Eddie Durie and lawyer Donna Hall, it was not used.

The officer who headed the operation, Detective Superintendent Larry Reid, said yesterday that it was "very, very person-intensive".

"That's the cost of law and order, that's the cost of investigation."

Up to 100 officers worked on the inquiry at times, ranging from beat constables to the elite special tactics group. The operating expenses included anything from vehicle costs to paying experts outside the police to assist the investigation.

Mr Reid said the operation was "not expensive" when compared with some major murder investigations.

A complex investigation such as the case of psychiatrist Colin Bouwer, who poisoned his wife, cost $1.2 million.

During the baby Kahu operation, Finance Minister Michael Cullen gave police the go-ahead to negotiate over the ransom demand, using money from the contingency fund used to cover unexpected events between Budgets.

The police told the Herald there was "no correspondence held on the file regarding the resourcing of the inquiry or the ransom demand".

Baby Kahu's kidnapper, Terrence Ward Traynor, a 54-year-old unemployed spray painter, is serving an 11-year prison sentence.

Baby Kahu, a "whanau baby" the couple adopted from Ms Hall's sister after losing their own baby, is now back with her birth mother in Rotorua.

Ms Hall has said the 16-month-oldis "fine, she is absolutely delight-ful".