Police investigating the murder of the Kahui twins showed "errors of judgment" and did not properly record evidence to the standard expected in such a significant and high-profile case, a police watchdog says.

An Independent Police Conduct Authority (IPCA) report into the police investigation, released last night, said lawyers for the babies' father, Chris Kahui, were placed under "unreasonable pressure" because they weren't properly informed about a witness statement potentially implicating the boys' mother.

In May 2008, Mr Kahui was found not guilty of the murders of his sons Chris and Cru. The infants died in hospital from serious head injuries inflicted in June 2006.

The witness, Eruera Tuari, a former lover of the mother, Macsyna King, told police in August 2007 that Ms King told him, "Chris didn't do it" and "I did it".

IPCA head Justice Lowell Goddard's report quotes police as saying they didn't disclose Mr Tuari's revelation because they wanted to complete other inquiries, such as speaking to Ms King again.

But this was not done, and the defence team did not receive the evidence until six weeks before the trial.

"The late disclosure put the defence under unreasonable pressure, and constituted an error of judgement on the part of the officers concerned," said the IPCA report.

Last night, Mr Kahui's lawyer, Lorraine Smith, said if the defence had been given the disclosure earlier it could have pursued more evidence against Ms King, whom it accused of being the killer.

"It would have made our job a lot easier," Mrs Smith said.

Crown prosecutors told the IPCA that they also did not know of the evidence until six weeks before the trial and were shocked by the police delay.

During the trial, Detective Sergeant Chris Barry said the delay was because police were too busy with a wave of violent crime to re-interview Ms King.

The report reveals a split between police and the Crown.

"Police have a different view of events," it says. "They say the existence of Mr Tuari's statement was disclosed to the Crown in November 2007; that the Crown instructed police to interview Macsyna King and then to disclose all of the related material to defence counsel."

Mrs Smith and co-counsel Michele Wilkinson-Smith alleged police sat on the information for six months, and suspected it was disclosed only because police thought Mr Tuari might contact the defence team during the trial.

Justice Goddard said that whatever the police view was of the credibility of Mr Tuari, "the credibility of witnesses is a jury issue" and should have been disclosed sooner than it was.

But the IPCA found no evidence police conspired or deliberately tried to pervert the course of justice.

Mr Kahui's lawyers also alleged that the date of a call from the cellphone of Ms King's sister Emily King - which Macsyna said she was using on the night the babies were injured - was left off a document police gave them.

The call registered on a Mangere cell site during the time the Crown said the babies were injured, when Ms King said she was in Henderson.

The defence lawyers claimed police did not record the interview with Emily King about the call and they were given no documents relating to it.

The IPCA said it was concerned by the omissions but believed they were not deliberate.

"However, the police failure to document the interview with Emily King was not in accordance with the standard of policing to be expected in such a significant and high-profile murder investigation."

Mrs Smith told the Herald last night: "Mrs Wilkinson-Smith and I had concerns about aspects of the conduct of the police investigation which became apparent during the trial.

"This report from the IPCA has justified those concerns and we are pleased with the report."

Mrs Smith said it was "incredible" Justice Goddard didn't know whether to believe the police or the Crown about when they were told about what Mr Tuari said.

Mrs Smith said the cellphone call was discovered on the second night of the trial and was critical.

"In the relevant time when the Crown says the babies received their injuries Mum was not in Henderson but the phone she said she was using ... that was in Mangere, where the twins lived."

The officer in charge, Detective Inspector John Tims, said he could not comment until he had seen the report.

Crown Solicitor Simon Moore, SC, who led the prosecution team, could not be contacted last night.

JUNE 18, 2006
Christopher and Cru Kahui die in the Starship Hospital.

OCTOBER 26, 2006
Their father, Chris Kahui, is arrested and charged with their murders.

MAY 22, 2008
A High Court jury finds Chris Kahui not guilty of murder.

Kahui becomes a father again after the birth of daughter Carla to fiancee Marcia Ngapera, but has strictly controlled visitation rights.

FEBRUARY 8, 2010
The inquest into the twins' death, due to be held on February 22, is postponed indefinitely.

An Independent Police Conduct Authority report criticises police actions.