A retired long-serving district court judge has criticised police for their treatment of television journalist Heather du Plessis-Allan, who is under investigation after purchasing a gun without a firearms licence as part of a news story.

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Roy Wade said he "very much" doubted whether there was a need for police to rifle through the journalist's private space and belongings, given the nature of the investigation.

Yesterday police executed a search warrant at the Wellington home du Plessis-Allan shares with husband, Newstalk ZB journalist Barry Soper.

The search came after an expose by du Plessis-Allan for TV3's Story programme. She collected a rifle from a courier after sending Gun City a mail order form in which the "Police Use Only" section had been filled in with a fictitious officer's name and police registration number.

The form included details of the credit card used to pay and a firearm licence number, apparently checked as genuine by Gun City staff.

Today Mr Wade, who presided over cases in the Auckland region, wrote to Commissioner Mike Bush expressing concern and "sadness" over the handling of the investigation and search warrant.

He revealed he first met Mr Bush 21 years ago when the then-sergeant arrived on his doorstep at 5am with a search warrant.

Mr Wade said that warrant was erroneous and appealed to Mr Bush to think hard about the decision to search du Plessis-Allan's residence.

"Search warrants are inherently obtrusive: how would any one of your staff (or you, for that matter) like to be subjected to seeing their most intimate belongings and documents pored over by total strangers for no reason at all?" he asked in the letter seen by the Herald.

Mr Wade said the police had done themselves "great harm" by their "insensitive treatment of the citizens of this country".

The Herald has asked Police National Headquarters for a response to the letter.

READ THE FULL LETTER BELOW