A North Shore officer leaked secret information from the police computer network to his wife in a bid to win a custody battle with her ex-husband.

An internal investigation has 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.

Mr Beatson 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, laid a complaint with the Waitemata police in August 2007. However, the matter was not investigated until January last year after he complained to the Privacy Commissioner and the Independent Police Conduct Authority.

Police have now confirmed Mr Beatson made unauthorised checks, but would not reveal what disciplinary action had been taken.

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

"In this investigation a thorough and robust process was undertaken, including examination by external parties of the circumstances and facts.

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

But he said details of the outcome remained confidential between police and the individual in accordance with employment law. Mr Searle said the security of personal information held by the police was "paramount".

He said use of the NIA was subject to random auditing every month and the rules governing its use were clearly laid out in police instructions.

"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."

The former husband of the woman said he was "totally disgusted" with how long it took police to investigate his complaint.

While he was pleased the internal inquiry found Mr Beatson had breached the code of conduct, he was still disappointed police were keeping some details secret. "The police have made things really difficult for me over this."

The Privacy Commissioner's office declined to comment yesterday. But the Independent Police Complaints Authority confirmed an investigation was held.

The National Intelligence Application is a computer network that holds information on people's criminal convictions and whether they are wanted by police or are a surveillance target.

It also keeps details on criminals' associates and their addresses.

In August 2009, figures provided by police national headquarters showed 33 staff had been caught making unauthorised checks of the NIA since August 2007. Nine later resigned.

A police spokesman said the figure was low given that the force had more than 11,500 staff and the public made 1.7 million phone calls to police communication centres every year.

Many unauthorised checks highlighted in the random audits were of an innocent nature on personal acquaintances, a practice which is banned. The audits were introduced after officers were caught looking at the personal file of a complainant in a high-profile police sex case.

- Additional reporting: Vaimoana Tapaleao