Police use of footage from high-tech automated number plate recognition cameras is being challenged in court by defendants.

At least 5000 cameras in two private networks provide footage of vehicle licence plates that police use to prosecute people.

At the heart of the unprecedented legal challenges is that this amounts to use of a tracking device without a warrant, in breach of search and surveillance laws. Another challenge is that it is in breach of the Privacy Act and the Bill of Rights.

There are at least two court cases, but suppressions mean details cannot be reported.

One argument is that the footage is obtained as part of a private contractual arrangement with a surveillance system paid for by the police.

The Criminal Bar Association says police's use of automated number plate recognition (ANPR) is just the "tip of the iceberg" of surveillance methods police were not being open about.

"We need to have a public conversation about the appropriate level of state surveillance in a free and democratic country," it says.

Police have had a partnership to tap into the private web-based ANPR network of Auckland company Auror for years, and also into the much smaller network of its rival SaferCities.

Six thousand staff can access the networks for routine non-tracking searches - that is, identifying the owner of a car in, say, a supermarket carpark or petrol station forecourt as a one-off - and 1000 of those have the extra authority to use the information for more invasive tracking of vehicles from place to place.

All in all, police officers use ANPR hundreds of times a day.

"It is an effective tool to support police in preventing and investigating crime in our communities and is regularly used to assist with serious investigations including homicides, assaults, and aggravated robberies," the police said in April.

Auror talked up its close relationships with various police forces, including in New Zealand, in its marketing of ANPR as a crime prevention tool to retailers including in Australia, the UK and US.

After police last year admitted misusing ANPR to track cars in a couple of Covid-19 cases, they did an internal audit of 350,000 uses.

This found five staff might have misused it, but that in the vast majority of cases police "can be trusted to use ANPR data responsibly".

Police use of ANPR was "not widely known and therefore barely debated", says president of the Criminal Bar Association Chris Wilkinson-Smith.

"The existence of some technologies are often only revealed retrospectively when they appear in criminal disclosure as cases are prepared for trial, ANPR as an example, this is only the tip of the iceberg.

"This hasn't been happening in the past 10 years."

Police databases, combined with AI, could "reveal the most intimate personal information".

"Police have been allowed to adopt technology under a broadly permissive regulatory background and dependent largely on internal supervision."

A column in the New Zealand Herald by Police Commissioner Andrew Coster in 2021 has failed to spark a continuing national conversation about surveillance, says Wilkinson-Smith.

"What are the appropriate boundaries for police intelligence collection and what trade-offs are we, as a community, prepared to make in the interests of safety?" Coster wrote.

Wilkinson-Smith says there was scepticism back then if police really wanted to engage, and though the barristers' association met with Coster in 2021, "our concerns about transparency and supervision remained".

Police say there has been one case where the admissibility of ANPR evidence has been challenged that has gone to a pre-trial hearing, and that the evidence has been ruled admissible.

"We are not able to comment further due to other ongoing court proceedings."

Auror has been also approached for comment.

-Phil Pennington/RNZ.