A Nelson cyclist has told a court that a police officer did a "hard u-turn" in his patrol car in one fluid movement, ramming his cycle and wedging it into a bank.

"I could hear the tyres squealing and the engine revving. You put your foot down in a police car like that, it's going to drift," Shaun Taylor said of the moments before the collision.

Taylor, who was not wearing a helmet, said he went over the bonnet of the police car and landed on the ground against the driver's door.

SHAUN TAYLOR: Bruised and dazed after being hit by Dunn's police car.

He was bruised and dazed after the incident.

The trial of the constable driving the car, Garry Dunn, opened in the Nelson District Court yesterday.

Dunn has denied a charge of assault using a police car as a weapon. He has also pleaded not guilty to a second charge of assault, after he pepper-sprayed Taylor when trying to arrest him.

Taylor told the court he was cycling home along Rocks Rd on February 10, 2009. Dunn, who was stopped in traffic at Richardson St, saw him and told him to get off his bike because he was not wearing a helmet.

Taylor said he got off his bike, crossed the road and started walking on the footpath. However, he was worried about being late for his job at the Hot Rock Gourmet Pizza Pasta Bar in Tahunanui. He got back on his bike about 100 metres on and resumed cycling on the footpath.

Taylor said Dunn told him to stop again by Magazine Point. He kept riding on the footpath before crossing the road and riding up to Dunn's police car, which was stopped on the side of the road with its lights on.

Dunn then told him he was under arrest for failing to stop.

Taylor said there was a bit "of a debate" from both sides. He acknowledged that he was unhappy about being stopped, as he was worried about being late for work.

"[Dunn] kept saying he wanted me to go with him, and I said I wanted to go to work."

He admitted under cross-examination that he was probably a bit agitated, but said he was calm with Dunn.

"I didn't understand why he was trying to arrest me for the fact I had no helmet."

Dunn then told him he was under arrest, he said.

When Dunn grabbed his arm to arrest him, Taylor said he stepped back and pushed his hand down. He said he didn't want to be arrested because he had to go to work, and was hoping he could change the situation to get a warning instead.

Taylor said Dunn then asked him if he had been pepper-sprayed, and he said: "No."

Fearing he would be sprayed, Taylor said he pulled his sunglasses down from the top of his head.

Dunn then sprayed him, which he said was painful and burning.

"My immediate reaction was to turn round and start running up the hill."

Taylor said he ran away from Dunn up a driveway before going back to get his bike.

He was disoriented, and started riding back towards Nelson before turning around. He said he could see Dunn's car ahead of him, heading towards the traffic lights.

Taylor said he noticed Dunn's car starting to move, and it then did a sharp u-turn and headed straight towards him, mounting the kerb and driving straight into his bike.

He estimated that the car was travelling at about 40kmh when it hit the front wheel of his bike. He said he did not see it slow down.

"I went across the bonnet and hit the bank."

Taylor said he felt dazed and bruised, and the car had hit his right leg.

After the incident, he was arrested and taken to the Nelson police station.

Crown prosector Mark O'Donoghue said the issue for the jury would be determining whether the force Dunn used as a police officer was reasonable and proportionate to the situation he faced.

O'Donoghue said police officers were allowed to use a level of force if the situation warranted it. However, the degree of force Dunn had used was excessive and unreasonable.

Dunn's lawyer Garry Barkle, in a brief opening statement, said Dunn had been a police officer for 21 years at the time.

Barkle said Dunn disputed some of the facts of what Taylor said took place, and the jury needed to look at the circumstances in which the pepper spray was used, as Taylor was resisting arrest.