Wellington mechanic Marty Harrison at Car Therapy said the cars coming in recently are in a worse condition than normal, but these could be just the tip of the iceberg.
"Some of them are going to be 18, 20 months that they haven't had a check.
"And depending how far they've driven, could have done [kilometres] and be a ticking time bomb ... and take out someone else."
Graeme Swan from the MTA said it was a scary situation.
"It's gravely concerning - especially when we've been through winter, we've got summer holidays coming up that we've got vehicles out there that potentially 18 months to two years since we've been inspected.
"And it's a real worry your tyres wear out in that time, brakes wear out, suspension wears out, lights - out of adjustment or blown - so a huge concern for the industry."
Swan said he is deeply worried that there could be an increase in fatal crashes where vehicle faults are a factor because of the amnesty.
"There's a very real chance that a vehicle could be involved in a serious or fatal crash now that that hasn't been inspected for some time.
"We've seen an increase in the number of vehicle faults in fatal crashes. And you'd have to say if a vehicle hasn't been inspected for a couple of years, chances are [this number] could go up."
Swan said New Zealanders have a poor safety culture with vehicle maintenance - people think "she'll be right" or fix things themselves.
Wellington man Derek - not his real name - owns two vehicles with expired warrants.
He was happy to take advantage of the extension and is not surprised lots of others are doing the same.
"People put it off as long as they can, warrants cost money, work costs money."
MTA is warning of "huge" wait times - possibly many weeks - once people realise the grace period is over.
Derek said he is ticked off about how long it is going to take for his bike to get certified.
"I have a motorbike, it needs a warrant, but it's six weeks before I can get it in to get a warrant - so that is annoying."
Graeme Swan from MTA said the government has not done enough to push the public to get warranted ahead of the deadline.
He said the six-month grace period was too long in the first place - something the industry has been telling the government since April.
Waikanae Station Tyre and Service Centre owner Paul Brimblecombe said inspections should have started again straight after lockdown.
"As soon as we were back on deck, pretty much in a business-as-usual situation where people were allowed to drive the vehicles, I think they should have switched warrant of fitnesses back on at that time.
"I think locking it up for six months, all it did was just push everything into a bottleneck - I mean six months of warrant of fitnesses, it's half the fleet for the year could potentially turn up on the one day."
And the free ride for drivers is over from Sunday as councils and the police - at their discretion - go back to pinging people whose warrants, regos or licences expired more than 28 days ago.
The fine for driving with no warrant or registration is $200 and it is double that for driving without a current driver's licence.
