AA welcomes technology but civil liberties lobby fears innocent people may be caught in surveillance.
Police are claiming crime-fighting and road safety wins from computerised camera technology that worries civil liberties lawyers.
In three months in South Auckland, the automatic number-plate recognition technology helped police to seize 15 stolen vehicles, take 180 disqualified drivers off the road and recover other stolen goods from a number of offenders.
The Automobile Association is welcoming the technology as a useful road safety aid.
But lawyers fear innocent people, such as a driver stopped on a Waikato road last week on an incorrect indication her car was unwarranted, may be caught in widespread surveillance that could expand to tracking their movements to build profiles of "people of interest".
That is denied by police, who say their use of the technology is governed by a manual containing strict guidelines and that "law-abiding motorists have nothing to fear".
The Waikato driver's partner told the Herald she had just renewed her car's warrant but the mechanic who issued it had not got around to updating a Transport Agency database.
The officer who stopped her said the car had shown up on his "computer screen" as having no warrant.
Police spokesman Ross Henderson said it was unclear whether the officer had one of five number-plate recognition units costing $40,000 to $50,000 in a long-standing national trial of the technology, or used a new iPad or iPhone to run a vehicle check.
But he said the technology was simply the automation of a process police would otherwise conduct manually by calling a radio dispatcher to obtain information already held.
The police have been revolving the five units around their 12 districts since 2009 in what remains a trial exercise, still under evaluation.
Each can scan up to 3000 number plates an hour, depending on traffic conditions.
Their operating manual warns officers against using them to hook up socially with strangers or record vehicle registration numbers of people attending peaceful protests.
Mr Henderson said the system was focused simply on vehicles, and did not store any form of facial recognition, as feared by Wellington civil liberties lawyer Michael Bott.
"It has been successfully implemented by enforcement agencies around the world and assists police to identify vehicles of interest without disrupting other road users."
But Mr Bott said equipment used by British police was capable of taking images of vehicle occupants, and helped them to build up profiles of "people of interest".
"With the net widening to take a photo of every car on the road automatically, every person is a suspect."
Auckland Council for Civil Liberties president Barry Wilson, also a lawyer, said: "The potential for tracking people's movements, spying on people, is just enormous."
Mr Henderson said the police were talking to the Privacy Commission after being assured by their own legal advisers about the adequacy of the operating guidelines.
But Mr Bott said they should have involved the commission from the start, rather than relying on in-house staff to assure people their rights would be safeguarded.
The commission has previously warned such technology has to be used "carefully" and expects to be updated on the trials.
* Automatic number-plate recognition technology
* Can scan up to 3000 plates an hour and automatically check them against police and Transport Agency records.
* Five units under trial nationally - cost $40,000 to $50,000 each.
Results from three-month trial in South Auckland
* 15 stolen cars seized.
* 180 disqualified drivers caught.