Crown prosecutor Tim Gresson reads his statement as (from left) Ian Howard, Simon Vincent, Alister Doonan and Clinton Vallender look on. Picture / Simon Baker

Crown prosecutor Tim Gresson reads his statement as (from left) Ian Howard, Simon Vincent, Alister Doonan and Clinton Vallender look on. Picture / Simon Baker

Witnesses to the Prime Minister's motorcade that allegedly travelled at up to 172km/h feared a deadly accident, a court has heard.

The witnesses gave evidence yesterday on the first day of the trial of five police officers and a civilian driver involved in the motorcade carrying Prime Minister Helen Clark and Agriculture Minister Jim Sutton on July 17 last year.

The six defendants face charges in the Timaru District Court including careless driving, dangerous driving and travelling too closely.

The Crown alleges that the police diplomatic protection squad (DPS) member in charge of the motorcade failed to set appropriate speeds and the drivers drove too closely and passed vehicles in the face of oncoming traffic.

The defendants had incorrectly interpreted the situation as "one of extreme urgency" and they had no legal right to travel in such a way.

The motorcade allegedly travelled the 205km from Waimate to Christchurch at an average of more than 128km/h to ensure that Helen Clark could catch a flight to get her to Wellington in time for a Bledisloe Cup rugby test.

According to prosecution statements, lead car driver Constable Simon Vincent acknowledged travelling at 160km/h to 180km/h on straights before Dunsandel, while the Crown said evidence would show security car driver Constable Ian Howard travelled at up to 172km/h.

Prosecutor Tim Gresson said Vincent had accepted that the vehicle he drove was prone to rolling on corners at high speeds. He had also said the ute's sirens were ineffective at such speeds.

South Canterbury farmer Rebekah Welsh told the court she was tending a sick animal on her property at Orari when she saw two cars led by a police car with lights flashing on State Highway 1.

Miss Welsh said the second vehicle appeared to pull out on to the wrong side of the road near a rise in the highway and she feared there could be a collision with an oncoming car.

"I immediately turned around. I didn't want to see it," she said.

She said she had never seen vehicles "travelling that close, at that speed".

Retired Christchurch man Brian Roberts was travelling with his wife, Shyrlee-Ann Roberts, near St Andrews in South Canterbury when they had to move left to make way for the two vehicles led by the police car with flashing lights.

"The cars keep coming very, very quickly so we all move over because it was obvious they were in the middle of the road," Mr Roberts told the court.