A group of vandals pulled the emergency brake on an Auckland train with commuters still in the carriages before causing $20,000 in damage.

Auckland Council paid $600,000 to controversial private investigators Thompson and Clark to catch graffiti vandals – but has no idea how many were prosecuted.

The Auckland private eyes, who are due on Tuesday to learn the outcome of a State Services Commission inquiry into their work spying on anti-government protesters, won a tender contest in 2013 for a five-year contract to take on the council's anti-graffiti investigations.

The contract required Thompson and Clark to send files on two recidivist and two 'emerging' taggers – ones with under 100 recorded tags – to police each month.

Neither the council, police, nor Thompson and Clark could say how many of those offenders were prosecuted, convicted or made to pay reparations.

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The council said it never logged that information. Police denied an Official Information Act request, saying it would be too hard to collate because files were lodged across three police districts. Later, Police changed its mind, but demanded a new request be filed.

SUPPLIED Train tagging has been in the headlines in recent years. Thompson and Clark tracked repeat offenders and supplied files to police.

Thompson and Clark boss Gavin Clark said he had a "rough idea" of numbers, but wouldn't comment on operational matters.

It's thought only a fraction of the 48 files per year were prosecuted. An industry source said the graffiti contract "had long been considered a bit of a joke, including the years Thompson and Clark have had it. The council don't keep track of what you do, they just pay you."

Of five years of annual reports on the company's work, the 2016-2017 report was missing and may not have ever been submitted.

One councillor, John Watson, attacked the lack of transparency and accountability over the deal, calling it "pretty loose". Watson said ratepayers deserved much more analysis of the spending.

Council deputy leader Bill Cashmore said it was actually Thompson and Clark itself that set the self-monitored investigation targets – even though they came with financial bonuses – so it could "prove to council they were actually doing what they said".

Cashmore said the council wasn't motivated by prosecutions, because most offenders were young teens who could be engaged with to change their behaviour and only a minority were older recidivists.

Prosecuting was a police decision that "I don't have optics over, and nor should I".

AUCKLAND COUNCIL Auckland Deputy Mayor Bill Cashmore.

Because of the relatively small size of the contract, Cashmore said it was council officials, not his committee, which had oversight of the deal.

Auckland Council's service and integration manager, Duncan McLaggan, said education and prevention work was a "far more significant" part of a $4m-a-year graffiti strategy that delivered a "pretty much graffiti-free city".

"Prosecution isn't the main outcome we are looking for – the main outcome is prevention," McLaggan said. "It's not actually a great outcome from our perspective to simply prosecute young people."

He said Thompson and Clark's job was a "very specific" one of targeting recidivists – but admitted the council didn't keep any record of those recidivists.

Cashmore also described the private eyes' role as a "very small component".

"In the big picture, we are seeing some good results," he said.

"There is a lot less graffiti around and there is a lot of young people getting the chance to get some life training."

AMY BAKER/STUFF Councillor John Watson said ratepayers deserved much more analysis of the spending.

But Watson slammed the council's dealing over the contract, saying: "Over such a prolonged period with such a substantial payment, you would expect some fairly extensive records going to the return on the ratepayer dollar … either convictions, or periodic detention, or youth aid or community service.

"The average punter would say 'how many were made to scrub the walls? What was the penalty?'

"I'd say the lack of accountability and clear transparency for such a large sum is disturbing. Where's the accountability and what's being delivered? Who has been the arbiter of any improvements? Who has done the analysis?

"For $600,000, you would expect thorough insight and analysis ... when no one knows what the records are, that's pretty loose."

While the contract talks about securing reparation from offenders, there was no mention of that in the annual reports. Clark said he was aware of "some" being paid.

McLaggan thought it was about $3500 a year, because of the young age of the typical offender.

The reports showed the company hitting its 48-file target in year one, and exceeding it with 72 in year two. It then deliberated reducing its efforts in year three, only meeting its target ("without making an effort to exceed it") to prevent the council having to pay out a performance bonus.

In its third annual report, a note from Clark said the company had "sort [sic] the outcome" of the files from police but implied it did not get a response.

ADVENTURE PICS Thompson and Clark boss Gavin Clark.

Asked about the suggestion only a fraction of files were pursued, Clark said: "At the end of the day, there is a variety of outcomes. A lot of these offenders are juveniles, and I don't think prosecutions are in the best interests. I think there are other outcomes."

Clark said the company had "revisited" specific offenders to see if there had been an impact and there had been "a fair amount of success".

Thompson and Clark, effectively co-owned by Clark and Nick Thompson, both former senior police officers, is awaiting the outcome of the State Services inquiry on Tuesday.

The company was paid by Southern Response, which was managing Christchurch earthquake claims for EQC, to spy on claimants; by government mining company Solid Energy to spy on environmental protesters, and by oil companies to spy on Greenpeace, which then shared that information with the Ministry of Business, Innovation and Enterprise.

The council renewed Thompson and Clark's contract earlier this year. Cashmore said council had worked with the company for some time and said the strategic procurement team would be "very sensitive" and have a "weather eye" on the inquiry.