An investigation by the Independent Police Conduct Authority has found that police failed to properly document and disclose some evidence prior to Chris Kahui's trial for the murder of his twin sons.
Mr Kahui was found not guilty of the killings in 2008.
The IPCA found the police did not reveal evidence to the defence about a mobile phone call. It says the failure to do so risked jeopardising the trial.
The report also says an error of judgment kept the existence of a crucial witness from the defence.
Eruera Tuari had said that the twins' mother, Maacsyna King, had confessed to killing them.
However, the authority says the police officers responsible did not deliberately try to pervert the course of justice.
Defence lawyer Lorraine Smith told Morning Report she expected the police would take disciplinary action against the officers concerned.
She said the omissions made the trial even more stressful.