An Auckland private investigation firm has been caught out after it attached a sophisticated tracking device to a political campaigner’s car – but left the device visible from outside the vehicle.

The GPS tracking device, which used a mobile phone connection to report the car’s position to private investigators, had been attached with magnets.

It is the third time in three years the Sunday Star-Times has caught Thompson & Clark Investigations doing covert surveillance on political groups for corporate clients.

On April 22 this year, animal rights campaigners Jasmine Gillespie-Gray and Rochelle Rees were in Levin, where Gillespie-Gray was in court for filming inside a chicken-processing farm.

The judge dismissed the case and later that day the pair noticed a black box under Rees’ car. When they removed it, they found the tracking device, a cross between a GPS receiver and mobile phone.

The Star-Times traced the device to Thompson & Clark Investigations, which had obtained the device from Auckland firm Argus Tracking Ltd, which advertises tracking services for companies to monitor their own fleets.

Thompson & Clark co-director Gavin Clark declined to comment on “anything we might do operationally”.

Rees said the campaigners were upset but not surprised at finding the device “given the past spying we’ve had to put up with from Thompson & Clark”. They were relieved they were able to spot it so easily: “Whoever put it there was incompetent, there’s no other explanation.”

She thought it was “very likely” Thompson & Clark was monitoring them for the Pork Industry Board.

Save Animals From Exploitation director Hans Kriek said he was “99.9%” sure the board was the client, because Gillespie-Gray’s group had been focused on visiting their farms and filming conditions.

Asked about the tracking, board chief executive Sam McIver said: “You need to ask Thompson & Clark”, but confirmed the board did get “generic” information from Thompson & Clark to protect its members. The board passes on information about animal rights groups to pig farmers.

Argus director Aaron Muir couldn’t confirm whether the unit was theirs, but said: “I don’t agree with tracking people’s vehicles anonymously… putting things on people’s cars if they don’t know about it.”

He said 99% of their business was tracking and fleet management of commercial assets and he hoped the ones sold to investigators were being used for “good purposes”.

“We don’t want to be known as the tracking company that tracks where someone’s wife goes, or something like that.”

Human Rights Foundation chief executive Peter Hosking said the device was an invasion of privacy and its use should be illegal. “State agencies would require a warrant to snoop like this, and there should be no lesser standard for private investigators.”

The legality of using tracking devices is questionable. Police must obtain a warrant before installing one on a vehicle and state in their annual report how many warrants they use each year – it was 22 in 2009.

A 2007 Law Commission report on police search and surveillance said that if private individuals used tracking devices then “presumably trespass-based torts would be applicable to the unlawful interference with the vehicle”.

That means private investigators using a tracking device without the vehicle owner’s permission risk civil action through the courts.

Wellington private investigator Trevor Morley said he would be worried if someone asked him to put a device on a private vehicle. “I almost definitely wouldn’t do it. It’s a very grey area… Sooner or later, someone’s going to go to court.”

Last year comedian Mike King, who was paid to promote pork products, visited a pig farm near Levin with animal rights campaigners and declared that the “callous and evil” practice of crate farming should be outlawed.

Since then the board has been trying to limit damage from its critics. A week ago TVNZ’s Close Up screened new footage of pigs filmed at the Levin farm.

The Star-Times has previously reported the pork board using Thompson & Clark to monitor animal rights groups.

In 2007, then Agriculture Minister Jim Anderton expressed surprise that the organisation, set up under legislation and funded by a statutory levy, would use private investigators. “It’s not the normal thing we associate with New Zealand institutions.”

In May 2007, the Star-Times revealed Thompson & Clark had been paying two students to infiltrate small environmental, peace and animal rights groups on behalf of state coal miners Solid Energy and other clients.

The students were paid to write reports on the campaigners’ meetings and plans, and to set up systems redirecting internal group emails to the private investigators.

Then Prime Minister Helen Clark said the surveillance was “unacceptable behaviour” and State-Owned Enterprises Minister Trevor Mallard ordered Solid Energy to stop.

In April 2008, Thompson & Clark was caught trying to recruit a man to spy on the same groups, including for Solid Energy.