The Independent Police Conduct Authority is urging the police to improve their handling of conflicts of interest, privacy and conduct while carrying out searches.
The authority has just released its recommendations, following a complaint from a Dunedin man two years ago.
Bruce Van Essen's home in Fairfield was searched in September 2006 at the request of the Accident Compensation Corporation.
In seeking the search warrant from the Dunedin District Court, Constable Andrew Henderson relied on information from Peter Gibbons, an ACC private investigator and his father-in-law.
In the report, Justice Goddard says Mr Henderson either should have been properly supervised or he should not have been dealing with his father-in-law at all.
The report concludes the search warrant lacked any clear description of offences Mr Van Essen was supposed to have committed, and may not have stood up in court had it been challenged.
Though no evidence of illegality or misconduct was found on the part of the police, the report says such behaviour runs the risk of undermining public trust and confidence in police.
ACC has so far declined to comment.
Mr Van Essen has indicated he will probably seek compensation over the matter, but is refusing to elaborate.
He says he is pleased the authority has upheld many of his complaints.
Other investigations defended
Dunedin police do not believe the report reflects badly on other investigations they have conducted.
Acting Southern District Commander Superintendent Malcolm Burgess says the IPCA report is incident specific, and does not believe the judgments can be connected to any other cases.
Mr Burgess told Morning Report on Monday that searches by and large are conducted efficiently, professionally and in accordance with the law.
He says police have learnt from the case in question and will do better in future.