A jury has been warned not to speculate about why a policeman is standing trial for the second time for an alleged assault on a Rotorua bar owner.

Ivan James Sarich, 36, is on trial after pleading not guilty to assaulting Glen Baker, the then part-owner and manager of Rotorua's Fuze City Bar on May 21, 2003.

Judge Neil McLean said Sarich, an Auckland policeman, was on trial for a second time because "something went wrong" at the first trial. But he warned jury members at the Rotorua District Court not to speculate on that and also urged them not to do their own investigations.

Sarich was with a group of police officers visiting Rotorua for a North Island police cross-country competition at the time and was among a group of eight who went to the Fuze City Bar.

After they had been there for some time a staff member refused to serve one of the group because he was intoxicated, Rotorua Crown prosecutor Amanda Gordon told the court yesterday.

A staff member, who saw the man being given alcohol by another in the group, told them they would be asked to leave if they kept giving him alcohol, the court heard.

Sarich's partner, Tracey Wong, asked staff to order them a taxi, and allegedly grabbed a large number of bottles of beer as she left. It is alleged she began to pass them to her friends who were sitting in the taxi and Mr Baker tried to stop her but she became abusive.

The bar's licence prevented patrons taking alcohol away from the premises.

Sarich allegedly became involved, grabbing Mr Baker around the neck, pushing him and punching him in the face.

The court heard the fracas lasted for about eight minutes and Rotorua police were called.

Mr Baker received scratches and friction burns to his neck, bruises on his arms and scratches on his back.

Ms Gordon said Sarich's and Mr Baker's versions of events were quite different, so the jury would have to decide who they believed.

Sarich's lawyer David Jones said an assault was justified if it was in self-defence or in the defence of another person.

"Think about what this man [Sarich] was thinking and what he was seeing as it unfolded," Mr Jones told the jury.

Mr Baker, who is now living in Australia, and a former staff member, now living in Holland, have travelled back to New Zealand to give evidence during the retrial, which is expected to take five days.