Senior police officers joked about framing Arthur Allan Thomas for the Crewe murders while drinking at a bar, says a woman who worked there.
Queenie Edmonds said she overheard the talk while working at the Mangere Hotel 43 years ago at the time of the investigation.
Ms Edmonds told One News that she heard police officers, including Bruce Hutton, who led the investigation, call Thomas a "half-wit".
She said police had bragged they could get away with anything.
"I thought it was idiocy that they could do that to another human being," Ms Edmonds said.
Harvey and Jeannette Crewe were shot dead at Pukekawa in 1970.
Mr Thomas, a local farmer, was convicted at two trials of murdering the couple before being pardoned. A royal commission later named Mr Hutton and another officer as responsible for planting the bullets that led to the convictions.
Police unsuccessfully challenged the finding.
The case is the subject of a police review after the daughter of the murdered couple asked Prime Minister John Key to help find her parents' killer. The findings of the two-year review are expected to be released soon.
Ms Edmonds said the bar had been reserved for a short time for police investigating the murders, and she had been sworn to secrecy.
"What I heard and saw in there was to stay in there," she told One News.
She also claimed to have seen the original murder suspect, Jeanette Crewe's father, Len Demler, drinking with the police inquiry team.
Deputy commissioner Mike Bush's eulogy at Mr Hutton's funeral last month caused controversy after he described him as a man of "great character".