Fears more than 21,000 cars are driving with dodgy WOFS

There are now more than 21,000 cars that could be driving around with a dodgy warrant of fitness.

All of those car owners are receiving vouchers to get them re-tested, but a road safety campaigner says there's no way workshops will be able to check them all in time.

In just 24 hours, another 5000 vehicles also have a question mark hanging over their safety - the latest suspensions being Super Cheap Tyres and Auto services in Auckland's St Johns, and El's Auto services in East Tamaki, where owner Elia Sipaia says he never even knew he was being investigated.

"I care for the safety of the people so I never have any complaints," Mr Sipaia told Newshub.

One of the car dealers who use his service says his business has stalled now that his cars need to be rechecked.

"They'll be losing sales; the customer comes in and says I want to buy this car, but if it's not ready in the yard, they can't sell it," Ajay Kumar said.

Road safety campaigner Clive Matthew-Wilson says many people won't bother to check their cars, and those that do might find it hard to find a garage that can do it before the voucher from NZTA runs out in two months.

The Transport Minister is calling it a case of systemic organisational failure.

"It's a huge concern to the Government, and I'm doing everything I can to hold the board of NZTA accountable to get this work done as soon as possible," Phil Twyford said.

How to know if your car's been affected:

- You'll get a letter and phone call if the NZTA has concerns about the WOF on your vehicle.

- If the car's been sold since the WOF was issued, the new owner will be notified.

- But if you've moved address or changed phone number in the last year, you need to update this with the NZTA.

- If your car needs to be restested but you have a crash in the meantime, the Insurance Council says insurers will still take the WoF as valid in any insurance claim.

- But they caution that if you don't get it checked again as soon as possible, you may put the safety of yourself and other drivers at risk.