Police have spent almost $200,000 reviewing their investigation into the Crewe murders, according to Radio New Zealand.
Farmer Arthur Allan Thomas was convicted of killing Jeanette and Harvey Crewe in June 1970 at two trials, but was pardoned in 1979.
The couple's daughter Rochelle Crewe, who was 18 months old when her parents died, had asked police to reopen the case. They refused to do so and instead decided to reinvestigate their original homicide inquiry.
According to RNZ, information released under the Official Information Act shows $195,000 had been spent on the review.
Most had been used to make digital copies of the original file of notes and evidence.
However, a large amount of information requested - including how much time staff had spent on the review, the number of people interviewed, and whether lawyers had been called in - had not been released by police, the broadcaster reported.