Retired High Court judge Sir David Tompkins QC is to oversee an investigation into South Auckland's police culture after a senior officer was convicted of assault and a judge said the police culture was "sick".

Earlier this month, Senior Sergeant Anthony Laime Solomona of Manukau-Wiri police was found guilty of assaulting a 17-year-old on the forecourt of a Manurewa service station in February last year.

Judge Bruce Davidson condemned Solomona's conduct, which also included photographing a 15-year-old boy wearing a sign that read "I am the property of Senior Sergeant Solomona".

Judge Davidson was unamused. "The practices may be reasonably widespread, especially in the south Auckland area. In my view, the culture is as sick as the joke," he said.

The Commissioner of Police Rob Robinson today announced Sir David would oversee the investigation into the culture in the Counties-Manukau Police District, assisted by Inspector Pieter Roozendaal from North Shore Waitakere.

In a statement, the Office of the Commissioner said the investigation's terms of reference were "to determine whether there is evidence of a police culture that condones or encourages acts of violence or other inappropriate treatment towards prisoners, suspects or other persons in the Counties-Manukau Police District."