5 The prosecution case was that Senior Constable Thomson was on duty as the driver of an unmarked police vehicle with Constable Hall. Whilst travelling south on the Port Wakefield to Adelaide highway, Senior Constable Thomson observed a vehicle travelling well in excess of the speed limit of 110 kph, north towards Port Wakefield. He observed the vehicle to be a high speed variant VT Commodore, either a SS or a Holden Special Vehicle. He made a U-turn as soon as practicable and pursued the vehicle at speeds of up to 190 kph. He took up a position to the rear of the vehicle and maintained a constant speed of 160 kph for a number of kilometres. He moved closer and followed and timed the vehicle over three-tenths of a kilometre at a constant speed of 160 kph. He then accelerated, activated his red and blue lights, flashed his headlights and pulled the vehicle over. The defendant admitted that he may have been travelling at 140 kph. Senior Constable Thomson formed the belief that anoffence had been committed and required the defendant to undertake an alcotest. As that proved positive he required the defendant to undertake a breath analysis test, which returned a reading of 0.101. His evidence was supported by Constable Hall.