Lee-Anne Cartier, 44, spent thousands of dollars on flights and phone calls searching for the truth about what happened to her brother, Philip Nisbet, and is now taking legal advice on possible action against the police.

"They did a half-arsed job," she said.

"There are consequences when you screw up."

Helen Milner, 50, on Thursday was found guilty of the murder of Nisbet, 47, in their Christchurch home on May 4, 2009.

Fairfax Media has learnt an internal police document details failings in the initial police investigation, which concluded Nisbet's death was suicide.

It is understood the report says Milner may have got away with murder had it not been for the work of Cartier and the coroner.

It also says:

- A senior front-line officer brushed aside concerns about Milner's unusual behaviour the day Nisbet's body was found, telling suspicious police constables it was suicide.

- Members of Milner's and Nisbet's family later confronted police with the belief she killed her husband, but detectives who investigated made basic errors.

- The file was not reviewed by a senior investigator before it was sent to the coroner.

Nisbet's family want changes to police procedure, requiring all sudden deaths to be investigated as homicides.

Police have refused to release any reports or documents prepared about the initial inquiry for privacy and employment reasons.

However, Canterbury District investigations manager, Detective Inspector Tom Fitzgerald, yesterday confirmed the police's initial investigation, which concluded that Nisbet's death was suicide, failed in several respects, but he would not comment about the internal document Fairfax Media has learnt of.

"We accept ... that some aspects [of the initial inquiry] were not handled thoroughly," he said.

"The initial investigation wasn't treated as a homicide as it should have been [and] many of the points you raise are as a result of that."

Officers involved in the initial inquiry had the errors brought to their attention but faced no other punishment, Fitzgerald said.

"I'm confident that they will never make those mistakes again."

He paid tribute to Cartier's amateur detective work, saying: "She did a fine job. Unfortunately she should not have had to have done that. I can feel for the families."

On the day of Nisbet's death, the first two police officers on the scene were suspicious about Milner's behaviour and the way events were unfolding, according to evidence in the trial. The internal report says they then raised their concerns with their supervising sergeant.

However, the sergeant told them the death was a routine suicide, the report says.

The decision meant a detailed scene examination was not carried out and valuable evidence was potentially missed or lost.

Days later, Milner's family raised concerns she had murdered Nisbet and detectives were assigned to the case.

However, after months of work they also concluded Nisbet's death was suicide and passed the file to the coroner.

Fairfax Media has learnt the internal document suggests the investigation had basic failings, including:

- There were not enough resources assigned to the inquiry.

- The home in Checketts Avenue was not searched after Nisbet's death, which meant evidence, including the box that had contained the fatal dose of Phenergan, was never found.

- A suicide note given to police by Milner was not treated as an exhibit and instead hole-punched and put in a folder. Fingerprints were not taken.

- Electronic equipment, including Milner's computers, was not seized to try to establish when the suicide note was written.

- Milner and Nisbet's text message records for the months leading up to the death were never obtained.

- Inquiries to establish where the fatal dose of Phenergan was bought were not thorough enough.

- Only a limited number of family, friends and work colleagues of the couple were interviewed.

Cartier's amateur detective work began after she became suspicious of her brother's death - particularly a suicide note she was shown by Milner.

She spent thousands of dollars on flights and phone calls gathering information.

Detective Inspector Greg Murton eventually picked up the file after Coroner Sue Johnson found no evidence Nisbet intended to commit suicide. He launched a murder investigation.

The inquiry team of at least six police had to build a largely circumstantial case, because potential physical evidence was not fully investigated at the time.

Family, friends and work colleagues of the couple were interviewed, financial, phone and email records examined and Milner's home searched twice before she was arrested on October 27, 2011.

Detective Superintendent Rod Drew, national manager of criminal investigations, said officers were required to treat all deaths as suspicious until proven otherwise.

If an officer had any concern about a death they should notify their supervisor and the police's Criminal Investigation Branch, Drew said.

"While police are careful to consider every death with caution it is not necessary to treat every sudden death as a homicide, and the resource required to do so would be prohibitive," he said.

Fairfax NZ News