A coroner's report has criticized police for a lack of adequate intelligence leading up to a two-day siege in Napier.
Jan Molenaar killed Senior Constable Len Snee, then wounded two other officers and a friend before turning a gun on himself in May last year. Police had gone to his home to conduct a drug search.
Coroner David Crerar's 39-page report says it is clear the presence of an extra police officer may have avoided the deaths.
But Mr Crerar says the main failing that led to the deaths was a lack of adequate intelligence about Mr Molenaar.
The coroner says his hearing earlier this year also identified deficiencies in the Arms Act and its enforcement, which he recommends be reviewed.
Mr Crerar has also suggested police digital radios be introduced more quickly, and that procedures for the execution of all search warrants by the police be upgraded.
Police have conceded officers underestimated the situation.
Superintendent John Rivers said police noted Mr Crerar agreed with Police National Headquarters's view that the reaction of Mr Molenaar to what was a routine search was extreme and disproportionate.
Any actions by police were not the cause of the death of Mr Snee but were incidental to it.
Mr Molenaar killed Mr Snee and then took his own life.
Mr Rivers said police were already acting on the recommendations made by the coroner. "Police will shortly be recommending policy changes to the minister concerning the Arms Act."
He said police wished once again to express sympathies to all the families involved in the tragedy.