A serious crash unit investigator believes a Holden Colorado was rotating clockwise on the highway when James Nelson collided with it on his motorcycle and died.

Constable Martin Krieger told the Whanganui District Court the driver of the ute, which was towing a trailer, lost control after two other motorcycles were travelling towards him on the wrong side of the road.

The two other motorcyclists, James Hiroki and Carey Leask, are on trial after pleading not guilty to charges of driving dangerously causing death.

As the second week of the trial got under way on Monday, Krieger estimated that he arrived at the scene of the accident near Ohingaiti at about 7pm where he began an investigation into the events that occurred on November 12, 2017.

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He said he found no fault with the stretch of road where the accident happened on State Highway 1.

"The environment can be fully discarded as a contributing factor in this crash," Krieger said.

Following the collision between the Colorado and Nelson's motorcycle, Krieger said it was most likely that the trailer towed by the ute came into contact with another vehicle.

Lawyer for Hiroki and Leask, Simon Hewson, asked Krieger if he looked along the roadside for tracks from vehicles that may have gone off the road.

Krieger said he had, but had not found tyre impressions from the Colorado, the trailer it was towing or another vehicle driven by Crown witness Daniel Goodwin.

Goodwin alleged that he watched the ute and trailer jack-knifing in his rear-view mirror when the driver lost control faced with motorcycles driving on the wrong side of the road.

Goodwin was travelling north to Hamilton with three of his friends when they were passed by a motorcycle, estimating it was 0.5-1m away from them at the time.

However, in his crash investigation report, Krieger said this was not a factor in the accident.

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Simon Hewson asked Krieger questions concerning the speed a vehicle can travel while towing a trailer, weight distribution on a trailer, following distance and reaction times.

Hewson said that in his testimony, Richard Hewson, the driver of the Colorado, stated that he was passed by two motorcyclists, side-by-side in their own lane.

Richard Hewson claimed a red van in front of him braked, forcing him to follow suit, pulling left to avoid contact and sending his tyres across the grass verge.

Simon Hewson asked Krieger to read a transcript of Richard Hewson's testimony, saying there was no evidence that he had ever turned right to correct his steering.

However, after reading the transcript, Krieger disputed this saying Richard Hewson had mentioned a right turn in reply to a question from Judge Bruce Davidson.

"The driver of the ute must have turned right to recorrect because it ended up on the east side of the road," Krieger said.

"If he had only turned left, the vehicle would have ended up on the west side."

Hewson also asked Krieger if it was possible that the trailer could have spun to the front of the ute, becoming the lead and continuing the momentum of the ute forward.

Krieger estimated it was possible in another scenario, but not in this one as the trailer weighed between 200-300kg and the ute was between two and two-and-a-half tonnes.

"The trailer's not going to say 'right, I'm going to Taihape' and just carry on going there," Krieger said.

The trial continues with Crown witness testimonies on Tuesday.