A police officer has described seeing a motorcyclist "riding like a twat" just hours before two Highway 61 gang members allegedly caused a crash that killed their friend.

James Eric Hiroki and Carey Raymond Leask have pleaded not guilty to charges of driving dangerously causing death and their trial began in the Whanganui District Court on Monday.

James Gregory Nelson died on November 12, 2017 after colliding with two cars while riding his motorcycle near Ohingaiti in Rangitikei.

The three were allegedly riding in convoy that day and Crown witness Sergeant Phillip Patterson told the court he was riding his personal road bike to work in Rotorua when he saw three Harley Davidsons.

Patterson said they were riding in formation and "all three of them were wearing gang patches on their backs. Highway 61 they said".

The riders were travelling in the same direction as Patterson and in a 15-minute period he said he observed the middle rider crossing the centre line on a number of occasions.

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Patterson said when the leading bike overtook other road users, all three would overtake "as a pack".

"As we were approaching Okere Falls, the road at that point has a right turn bay the opposite direction and a lane next to that. The centre line is a solid yellow no passing line.

"The rider in the middle, for I don't know what reason, pulled across the line and drove through the median that was for the right turn."

Patterson estimates he saw them pass six or seven vehicles.

Judge Bruce Davidson
Judge Bruce Davidson

At one point on a two-lane road "he pulled straight into the left without indicating in front of a Landrover Defender".

"The guy thought it was his road. I was surprised the Defender didn't ride right over the top of him."

When Patterson arrived at work, he looked up the registration number of the motorcycle, made contact with police to keep an eye out for it and filed a report.

"The middle rider was riding like a twat. Made a left turn from the right side of the road and cut off another vehicle," it said.

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The lawyer for Hiroki and Leask, Simon Hewson, suggested Patterson had not identified the riders were speeding.

He agreed and said at one stage he had actually caught up to them.

In his opening address, Crown prosecutor Chris Wilkinson-Smith said the two defendants and Nelson were first seen driving as a group in Rotorua.

The Crown alleges earlier that day Hiroki also came into contact with another car after he had crossed the centre line at Bulli Point near Taupō.

Later that day near Ohingaiti, a vehicle towing a trailer lost control when the driver reacted a scene ahead and crashed into Nelson's motorcycle and another vehicle.

The trial before Judge Bruce Davidson is expected to take five days.

Hiroki also faces three charges of driving dangerously causing injury, two charges of failing to stop or ascertain injury, possessing an offensive weapon and driving dangerously.

Leask is facing three further charges of driving dangerously causing injury and failing to stop or ascertain injury