Bozzone is an accomplished triathlete and has also competed in duathlons and track and field events.

Triathlete and Ironman champion Terenzo Bozzone has been seriously injured after being hit by a truck.

Bozzone was cycling on Waitakere Rd in Kumeu, in Auckland's northwest, when he was hit just after 2pm on Tuesday.

A witness who was first on the scene said they saw the truck pull over on the side of the road, then drive off at speed.

"When the truck drove off I saw a man lying on the road. I jumped out to help him; he was semi-conscious and his bike was in two pieces."

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The witness realised the cyclist was Bozzone after his father showed up at the scene.

GETTY Bozzone in action in Hawaii in 2017.

"I left as the paramedics were loading [Bozzone] into the ambulance. He was conscious but seemed to be in a lot of pain."

A St John Ambulance spokeswoman said a person had been taken from the scene of the crash to Auckland City Hospital in a serious condition.

An Instagram post uploaded to Bozzone's account on Tuesday evening said the athlete was in the emergency department, where his injuries were still being assessed.

The post asked anyone with information about the crash to contact Henderson Police.

About 9pm, an Auckland District Health Board spokeswoman said Bozzone was in a stable condition.

The witness described the truck as a brown logging truck with metal plates.

GETTY Bozzone in 2017 at the Ironman 70.3 Middle East Championship in Bahrain.

A spokesman said police were "making inquiries" to speak with the driver.

Emergency services were on the scene for about three hours and motorists were being advised to avoid the area or allow extra time.

The road was closed for a few hours but was re-opened about 5.15pm.

The serious crash unit was investigating.

Bozzone, 33, moved to New Zealand from South Africa as a young child.

In March, he won Ironman New Zealand in Taupo in a record race time of seven hours, 59 minutes and 56 seconds, smashing the previous record by more than seven minutes.

He had previously come second on four occasions.

Bozzone also holds the course record for California's Wildflower Triathlon, which he set in 2006, and and in 2008 won the Ironman 70.3 World Championship in Florida.

He and his wife Kelly have two children – a son, Cavallo, and a daughter, Zaya. They live on Auckland's North Shore.

Ironman NZ spokesman Andrew Dewhurst said he was not aware of the incident.

He did not wish to comment except to "send our prayers and best wishes to [Bozzone] and the family", he said.