A police officer who leaked secret information to his wife in a bid to win a custody battle with her ex-husband also breached privacy law, a watchdog has ruled.

The Herald revealed in February that an internal police investigation found Senior Constable Terry Beatson used the police National Intelligence Application (NIA) to open the man's file 17 times over four years.

Doing unauthorised checks over the network is considered serious misconduct under the police code of conduct and can be grounds for dismissal.

The North Shore officer has kept his job despite giving confidential information to his wife, who was in a custody battle with her ex-partner over their young son.

The former husband discovered the leak when he found private details were contained in an affidavit his ex-wife filed with the Family Court.

The man, who asked not to be named to protect his son, does not have a criminal record.

Now, the Office of the Privacy Commissioner has ruled the police breached two principles of the Privacy Act.

Given the "acrimonious relationship" between the two men, assistant Privacy Commissioner Mike Flahive said police failed to take reasonable steps to ensure the security of the personal information.

In a second breach, Mr Flahive wrote in his decision that Mr Beatson used the information for a purpose it was not intended for.

"It is my view that you have suffered harm of this type based on the fact that Senior Constable Beatson has used his privileged position within police to access your NIA records primary to promote his interests over yours," Mr Flahive said.

The case has been referred to the director of the Human Rights Proceedings, who may take a case before the Human Rights Review Tribunal.

Police have not revealed what disciplinary action was taken against Mr Beatson.

The Waitemata district commander, Superintendent Bill Searle, confirmed police had conducted an investigation into inappropriate access of the NIA by a staff member.

"Strong disciplinary action has been taken - short of dismissal.

"Any allegation of inappropriate accessing of personal information on police systems is taken extremely seriously, and is investigated in accordance with the police code of conduct."