The investigator hired by the Exclusive Brethren to tail Prime Minister Helen Clark assaulted a man in custody and kicked a senior police officer in the groin during a bar-room brawl 25 years ago.

Herald files show private investigator Lew Proctor was convicted of assaulting three officers and another man after going berserk during a drinking session at the Papakura Police Station bar when still in the force as a detective in 1982.

The Herald files show Proctor struck one constable, headbutted another constable in the face and intentionally kicked Inspector Barry Davies in the groin as well as intentionally damaging a door.

Proctor's rage began after a heated conversation about dogs with other officers in the bar, where he was saying he wanted to "show how he had hit the black Samoan bastard who hit his dog".

He started taking several swings at a constable, putting his fist through a door, smashing a door and putting his fist through a toilet wall.

He then punched one of the constables in the face several times, butted the other and lashed out at Mr Davies, kicking him in the groin and later in the backside.

At some point, Proctor went into an interview room where a man was being held in custody, asked him a few questions, then punched him twice about the head.

In his defence, Proctor claimed he was suffering temporary insanity.

The sentencing judge said the assaults were out of character and unlikely to be repeated and imposed a fine of $600 to give Proctor the chance to start a new life.

Proctor could not be reached for comment last night but has previously told a newspaper that "his slate was wiped clean".

The officer he kicked in the groin, Barry Davies, went on to head the Tasman police district and retired in 2000 after 40 years in the force.

"I saw his face on the news and thought, 'Hey, I remember you,'" Mr Davies told the Herald last night.

"I've got no comment to make, though, really. He left the force, he got a private investigator's licence and here we are today.

"It was 25 years ago."