The Independent Police Conduct Authority says it recommended in April 2011 to the Commissioner of Police that the case of a missing Tauranga woman be further investigated.
Police announced on Wednesday they were offering up to $50,000 for any information about Luana Williams, 25, who disappeared from her Munro Street home in 1986.
Investigative journalist Bryan Bruce told Radio New Zealand's Morning Report programme on Thursday that he had serious concerns about the way police handled the case and what they did with information he had uncovered.
"I tabled a very serious complaint that suggested that two of the officers who were assigned to Luana's case had had sexual relations with her.
"Now that's an allegation, but it isn't for a journalist to go and investigate. This is an allegation that has to be investigated by the police."
Mr Bruce said he also complained to the Independent Police Conduct Authority, but had never been contacted by it to give more information.
But the authority said certain aspects of a complaint laid about the police investigation were looked into and the complainant informed of the outcome.
It also said police had been reviewing the case and it had deferred its investigation until the criminal investigation was completed.
Police said they had corresponded with Mr Bruce asking him to share any information he had.