The Independent Police Conduct Authority has launched an urgent inquiry into whether the police should have caught serial rapist Malcolm Rewa much sooner than they did.
The move has been welcomed by the brother of one of Rewa's victims but he is questioning why the investigation will not include the man he believes was wrongly convicted of his sister's murder.
One of Rewa's victims this week revealed she gave police his name in 1988 but they failed to check out his alibi.
Rewa is in prison for sexually assaulting 25 women over an eight-year period. Two juries could not decide whether he murdered one of those women, Susan Burdett, and another man, Teina Pora, is in jail for that crime.
Susan Burdett's brother, Jim Burdett, says whether Rewa could have been caught earlier than he was is a theoretical point - and the real question is whether a man has been wrongly imprisoned for 20 years for murder.
"Now that's a matter of substance, it's not just an interesting, hypothetical point. Without denigrating the rape victim's complaint, it's more important at this juncture to establish who did what, so the responsibility can be sheeted home to the right people."
Mr Burdett believes Rewa, not Pora, killed his sister. But authority says it will not investigate Pora's conviction.
However his lawyer, Jonathan Krebs, says something might emerge during the inquiry that his team can use in their appeal to the Privy Council in London.
Criminal Bar Association president Tony Bouchier says the authority should keep an eye out for evidence which might be used to clear Pora's name.
Police will fully co-operate
Police say they will fully co-operate with any investigation and will leave it to the authority to determine if mistakes were made.
Acting Police Commissioner Viv Rickard says rape complaints are dealt with differently now than they were 20 years ago, with system improvements, more resources and better victim support.
He says police take allegations of rape seriously and commit their best investigative and forensic resources to their investigations.