Commodore Alfred Keating was a senior defence attache at the Washington embassy when a covert recording device was found in a unisex lavatory in July last year, Judge Grant Powell said.

"It had been purposely mounted inside a heating duct in the bathroom at a height and direction that captured recordings from people who arrived and used the toilet," he said in a written judgement released Friday.

The hidden camera was discovered when it fell to the floor and a thick layer of dust on its mounting indicated it had been in place for many months.

While Keating had diplomatic immunity in the United States, police in New Zealand executed a search warrant on his home seeking evidence in the case.

No indecent images were discovered but police found Keating had installed driver software for the camera.

They also matched his DNA to samples found on the memory card in the camera.

Keating was charged with attempting to make an intimate visual recording in March and subsequently resigned from the military.

The High Court rejected a name suppression bid by Keating which argued he and his family would face "extreme hardship" if his identity was revealed.

Before the posting to Washington, where he was New Zealand's most senior military officer, Keating was assistant chief of navy in Wellington.

He has pleaded not guilty and the case is ongoing.