Eight cars have been stolen from VTNZ testing stations in the past 25 years, the Herald can reveal.
They include the most recent case on Monday, when the keys of a $15,000 Mazda were left in it while an inspector at the Westgate branch went inside to write out a WOF sticker.
Motorists are now threatening to boycott VTNZ testing stations if changes aren't made to prevent further thefts.
VTNZ has admitted the theft on Monday should never have happened and the responsibility, including covering costs to the owner, now lies with the company.
"When our customers leave their cars with us, they become our responsibility, and we take that seriously," country manager Greg O'Connor told the Herald.
"The keys should not have been left in the car once the inspection had been completed – if they had been removed, then this probably would not have happened."
Pressure is now growing for VTNZ to make serious changes, including installing security cameras as a deterrent to thieves.
Some motorists have gone as far as threatening on social media to boycott the testing stations until changes are made.
Ashlee Young's 2014 hatchback, rego LEGZII, was stolen at about 2pm while she waiting for the inspection to finish.
The branch, like many others, doesn't have any cameras so there is no footage of the man who is believed to have waited at the end of the workshop before taking the car when the inspector went inside to write up the WOF sticker.
Since running her story the Herald has been contacted by numerous people who say they feel uncomfortable about leaving their keys in the car but feel pressured by staff to do so.
Paul Thomas said he was even told he would be refused a WOF at the Parnell branch if he didn't leave his keys inside.
Motorist David Thew said he had complained about the practice at the Silverdale branch numerous times but never heard back from anyone after filling in the required forms.
"They almost always leave keys in cars when finished at Silverdale, out front of the building out of sight of the waiting room."
Another man said he was left feeling "on edge" after his car was parked out of view from staff after a WOF inspection at the Kingsland branch last week.
"I think it's common practice for VTNZ to leave keys inside customers' cars after inspection which is unprofessional and semi high risk of it being stolen."
A former VTNZ inspector told the Herald cars being stolen was a known issue and his branch had a sign put up stating people shouldn't leave their keys inside after one was taken from another North Island branch about three years ago.
He said staff were also warned about it - but it still happened anyway.
"They were encouraged to leave keys in ... I had heard on a few occasions people asking the assistant manager and his reply was, 'Just leave in ignition ... this isn't Auckland'."
The man said cameras needed to be installed as a deterrent.
VTNZ has since taken responsibility for all of Young's costs and paid for a rental car but she says changes need to take place.
"It seems ludicrous for it to be okay to leave keys in the car when it's unattended. I don't want this to happen to anyone else either," said the 26-year-old car owner.
"If VTNZ change their policy they'd save themselves a lot of potential strife if it were to happen again."
O'Connor said VTNZ was looking into what happened and how staff responded but would not comment on whether cameras would be installed.
"Where changes are needed, we will make them."