A police officer who made a fatal u-turn in front of a motorcyclist was left so badly shaken by the man's death he could not key in an emergency call on his cellphone, says a witness.

In an ironic twist, the allegedly speeding motorist who the officer was about to pursue when he made the u-turn may have been the motorcyclist's best friend.

Father-of-two and former motorsport champion Paul Brown, 38, was killed as he came over the brow of a hill near Waerenga, in Waikato, and crashed into the turning patrol car, which had its sirens on and lights flashing and was about to chase a ute that police say had been clocked travelling 154km/h.

Four investigations into the death of Mr Brown were under way last night, but police said the ute driver had not come forward and they did not know who he was.

However, Carl Jackson, of Jackson Engineering, where Mr Brown worked part-time, told the Herald Mr Brown had been following his ute as the pair headed to their respective homes after spending time at a friend's house on Sunday afternoon.

When asked if he had been speeding, Mr Jackson said he "might have been".

He said he didn't know if it had been him police had started to chase, but as he returned to town he "saw the ambulance speeding past" and figured something had happened.

One of the first people on the scene, neighbour Frank Wilkin, said he raced outside after hearing a "horrific boom" and saw a police officer and another man get out of the patrol car.

"They looked very, very shocked, they didn't know what the hell they'd hit. They were as white as sheets."

Moments later, the police officer raced into Mr Wilkin's house asking to use his phone because he was unable to use his mobile phone. The officer could not immediately see Mr Brown's body, which was flung about 30m off his bike and into a ditch, Mr Wilkin said.

"He had a cellphone but he was shaking so much he couldn't press the buttons. He was yelling, 'I can't find him, I can't find him'."

Mr Wilkin said there were "bits and pieces" of Mr Brown's motorbike strewn for metres across the road. Half of his helmet looked as though it had been crushed. The back of the police vehicle was "virtually torn off".

Mr Jackson condemned the police officer's actions. "It was a stupid place to turn around. If he'd gone another 100 metres he could have turned around safely."

He described Mr Brown as "one of my best mates".

Mr Brown was divorced and had two teenage children, Clayton and Chanelle, who lived with their mother.

Dozens of friends and relatives flooded the home of Mr Brown's parents, John and Jenny, in the nearby village of Waerenga yesterday. It is understood the couple lost another son in a motorbike accident years earlier.

Some people flipped through a photo album of images of Mr Brown racing. Mr Brown was the New Zealand Supermotard champion in 2005, but he hurt his back in a race in Wanganui in 2008 and had not raced competitively since.

He was a member of a group of biking enthusiasts called the Coro Demons and had their logo on his bikes, of which he had about four.

Waikato road policing manager Leo Tooman yesterday said the CIB, Serious Crash Unit and the Independent Police Complaints Authority were all investigating the crash. The coroner would also carry out an investigation.

Mr Tooman said police investigations would be supervised by a senior officer from outside the district with peer review by a crash investigator also from outside the district.

"Road policing is all about preventing road trauma. Unfortunately the nature of our business is such that from time to time tragedies occur. It is now a priority for us to ensure a thorough investigation is completed, not only for the deceased's family's peace of mind but for that of the public and our own staff."