A Christchurch motorist accused of speeding has taken on the police with his own technology and won.

Trained driving instructor Geoff Wilson believes he struck a blow for all motorists yesterday when he won an appeal against his speeding fine in front of Justice Lester Chisholm in the High Court.

"I knew I was right, and I knew someone had to stand up and say enough is enough," Mr Wilson said.

Mr Wilson used data from the global positioning system (GPS) in his car to counter the police charge that he was travelling at 113km/h when stopped near Parnassus in North Canterbury on January 16.

He conceded he may have got up to 107km/h, but claimed his GPS data proved he never crossed the 10km/h threshold allowed by police.

He also argued that his evidence showed police had wrongly identified his car as the one picked up for speeding by their stalker radar.

Mr Wilson was originally found guilty of speeding and fined $80 in the Christchurch District Court, but the justices of the peace hearing the case said they were not qualified to "either accept or reject" the GPS data.

In the High Court at Christchurch yesterday, Justice Chisholm said the fact the JPs did not seriously try to grapple with Mr Wilson's evidence made the conviction against him unsafe.

The judge also said there was doubt about the police radar coverage of the stretch of road Mr Wilson travelled.

Canterbury police's acting road policing manager, Trevor Pullen, said he could not comment on the court judgment as he had not seen the judge's finding.

Mr Wilson said until his appeal, no one had been able to challenge the integrity or ability of police to positively identify a vehicle in the scope of the radar.

"The courts always found in favour of police thinking these [speed radars] are infallible. I'm not against stopping speeding drivers. It is the poor innocent guy who gets caught up that I'm concerned about."

Automobile Association motoring policy manager Jayne Gale said the association would look at the High Court judgment with interest.

But she cautioned against motorists getting false hope that they could also beat their speeding fines.

"We are not finding many flaws in the police system."