A top Auckland policeman caught speeding yesterday told the officer who stopped him he was tracking a vehicle of "operational interest" at the time.

Superintendent Ted Cox was handed an infringement notice after he was clocked driving at 120km/h on Auckland's Southern Motorway.

Police national headquarters spokesman Rob Lee said Mr Cox was off duty driving an unmarked police car he used in a personal and professional capacity when he was stopped on the motorway between the Princes St and Highbrook Road exits in the Otahuhu-Otara area about 2.40pm.

"He told the officer he was catching up to a vehicle he believed was of operational interest to his work," Mr Lee told NZPA.

"He was served with an infringement notice and an infringement of that type generally incurs a $120 fine and 20 demerit points."

He said the officer who stopped Mr Cox acted in the right way as did Mr Cox.

"Mr Cox now has the opportunity to challenge that notice if he has a lawful defence."

Mr Lee said Mr Cox's car did not have the flashing lights activated when it was stopped.

Though Mr Cox was off duty at the time, Mr Lee said police staff were generally "still police officers 24 hours a day".

Mr Cox was stopped during a crackdown on speeding that aims to cut the number of deaths on New Zealand roads.