Police made mistakes in the Malcolm Rewa rape case, but there is "insufficient evidence" to say that prevented them catching him sooner, the Independent Police Conduct Authority says.
The police watchdog released its report into police's investigation of the serial rapist today.
The IPCA announced last August it would investigate the case after TV3 current affairs programme 3rd Degree revealed police failed to check Rewa's alibi after a woman, said to be his first victim and referred to as Ms A, told police his name in 1988.
Among Rewa's convictions is one for the 1992 rape of Susan Burdett. Two juries could not reach a decision about whether he murdered her.
Meanwhile, Teina Pora has served more than two decades in prison for raping and murdering Burdett. In January he was granted leave to appeal the convictions to the Privy Council.
Police have apologised in the wake of the IPCA report.
Assistant Commissioner Malcolm Burgess said police accepted the findings, and apologised for the identified failings.
He said it "remains a matter of significant regret" that police were unable to catch Rewa earlier and prevent future offending.
"We also apologise to Ms A, who police clearly failed at the time by not properly investigating information she supplied identifying Rewa," he said.
He said police staff investigating some of the earlier complaints did not meet expectations of policing at that time.
"We accept and apologise to victims affected by those failings."
Tonight Burgess told TV3 he hadn't ruled out a personal apology to Ms A.
"That's certainly something I'd give consideration to."
It also noted the paper-based record system of the time did not promote sharing of information across districts.
Burgess said with the benefit of hindsight, it was clear where linkages could have been made and lines of inquiry pursued.
"Today, any offending of a sexual nature is referred to suitably trained and experienced police officers who are specifically responsible for investigating adult sexual assault complaints and providing victims with access to support services."
Ms A was disappointed with the IPCA report, 3rd Degree reporter Paula Penfold tweeted.
"Also has had no apology from the police despite their public statement."
The IPCA considered four complaints it received about the police handling of the series of sexual assaults committed by Rewa.
The report focussed on the nature of the police investigation into the historical allegations of rape raised by a number of women, and whether police should have identified the perpetrator during the course of their investigation.
IPCA chair Judge Sir David Carruthers said the report found police failed to properly investigate information implicating Rewa as the offender for the sexual assault of the woman, referred to as Ms A.
In particular, the report said, police did not obtain a statement from the woman following her attack in 1987 regarding her naming Rewa as the offender. Nor did police attempt to make any inquiries that might have called into question the alibi given by Rewa at the time.
"The [IPCA] is unable to determine whether, if a proper investigation had been undertaken at the time, Mr Rewa would have been arested and successfully prosecuted," Carruthers said.
The IPCA noted media coverage of the case which suggested that police overlooked Rewa's involvement in six incidents, including three sexual assaults, and failed to take action.
"The authority found that there was no action that police could have taken in respect of any of the different types of offences which occurred in London Street that would have identified Mr Rewa earlier as the serial sex offender,"
Rewa was arrested in 1996 and convicted in 1998 on multiple charges relating to the sexual assault of 25 women spanning almost a decade.
He was sentenced to a term of preventive detention with a minimum non-parole period of 22 years.
He was later found guilty of the rape of Burdett, earning him an additional 14-year concurrent jail term.
Pora advocates, headed by private investigator Tim McKinnel in 2009, have been battling to clear his name of crimes they believe were committed by Rewa.
Carruthers said to note the report did not consider Pora's convictions, as "this is outside the jurisdiction of the authority and is currently before the Privy Council".
The IPCA identified various aspects of police strategy relating to handling of sexual assault matters that did not meet expectations of policing at that time, the report said.
"There have been significant practical, scientific, and technological advances in policing since that time, therefore it would be unfair to compare police actions of that era with the standards of today," Sir David said.
In 1994, aged 17, Pora was convicted for the rape and murder of Burdett, 39, in her Papatoetoe home two years earlier.
At a retrial in 2000, he was again found guilty, and sentenced to life imprisonment.
He spent 21 years in jail before being granted parole on March 31 this year. He is set to appeal his case at the Privy Council in November.
His lawyers have argued there was no direct evidence linking Pora to the scene, saying he was convicted largely because of a false confession. He has since been diagnosed with foetal-alcohol condition.
Burdett's older brother Jim has said he believed Pora is innocent.