KEY POINTS:
A 20-year-old motorcycle sidecar passenger is fighting for his life after the machine he was riding on ploughed into a safety barrier at the Pukekohe Park raceway.
Jack Thompson was the passenger, or swinger, on a machine being ridden by his father, John Thompson.
The father and son team were competing in the first sidecar race of the day when the accident happened, shortly before noon yesterday.
A witness said it appeared the bike suffered a mechanical fault as it spun out of control heading into the main straight.
"It came over the hill into the main straight and veered sharply to the right," he said.
"The rider tried to straighten, but the bike spun round and into the barrier, with the sidecar taking the brunt."
Thompson snr walked from the wreckage, but his son was taken by ambulance suffering multiple injuries.
He was in intensive care with head injuries at Middlemore Hospital today, his parents at his side.
Sergeant John Yearbury of Pukekohe police said early signs suggested "a mechanical fault or failure".
"The father seemed uninjured. He got up and was walking around but was in shock.
"The son ... seemed to have extensive leg injuries."
Mr Yearbury said police had taken video footage and details from witnesses.
Officers also took the bike away to check for mechanical problems.
Motorcycling New Zealand chief executive Paul Pavletich said the sidecar race was part of the fifth and final round of the national championships.
It was not known how long the father and son team, who are from Waipu, had been riding together.
Mr Pavletich said safety was "absolutely our number one concern" but it was too early to say what caused the crash, which occurred during the day's third race.
The track was closed for about an hour and a half after the accident.
Two years ago, novice motorcycle sidecar racer Karl Barritt, 22, was killed on the last lap of a race at Rosebank Speedway in Auckland.
Two sidecar riders have also died in the past four years in crashes at Ruapuna Speedway, Christchurch.
