Murder accused John Skinner told his partner he fired shots at Sergeant Don Wilkinson because he thought he was "reaching for something to harm him with", an Auckland jury has heard.

Tina Preece said Skinner, 37, and co-accused Iain Lindsay Clegg, 33, came into into their Hain Ave, Mangere, house after a confrontation with Sergeant Don Wilkinson and a colleague, Officer M, who had been attempting to install a tracking device on a car connected to Skinner, an alleged P dealer, on September 11, 2008.

Wilkinson and Officer M were gunned down when the operation was aborted after the occupants of the house emerged and chased them in a car. Wilkinson died at the Mangere scene. Officer M was badly wounded but survived the shooting.

Skinner and Clegg are on trial for Mr Wilkinson's murder at the High Court at Auckland. They are also accused of the attempted murder of Officer M and Skinner is charged with assault with a firearm.

Both men have pleaded not guilty.

Ms Preece said Skinner and Clegg changed the shirts they were wearing and washed their hands after they came into the house.

She told the court Skinner and Clegg then talked about what had happened. Skinner was "running around, gathering things" and wouldn't sit still. She described the scene as chaotic.

"He was talking to Ian. They were discussing who the people were. They were unsure who they were and discussing who they could have been."

They first came to the conclusion they must have been car thieves then thought they could have been planning a home invasion.

It wasn't until after Skinner received a phone call the possibility the men were police was raised, she said.

The witness said she was told they had confronted a man in the street and "bailed him up asking him what he was doing" and that Clegg had assaulted someone with nunchakus.

"That's when I found out that John had been the one who fired the shots."

When she asked him why he did that he told her he "thought he was reaching for something to do harm him with".

Crown Solicitor Simon Moore SC asked her if Skinner said how many shots were fired and if he indicated how many people he'd shot.

Her reply was he didn't say how many shots were fired and he told her he'd only shot one person.

Ms Preece said she knew there were BB guns in the house and Skinner had test fired them, using cans as targets.

The trial continues.