Owner of Advanced Auto, Alex Johnston, said one-in-three vehicles he dealt with would fail a Warrant of Fitness (WoF) test.
"Most of them I'd say were around 1990-2003."
Vehicles usually failed due to tyre and steering rack problems and worn brake disc pads, he said. "[It's] not usually a lot of structural stuff, cars in Wairarapa don't really seem to rust that much - not like over the hill."
Older cars were no different to newer vehicles if well maintained, Mr Johnston said.
The cars in the worst condition were those modified by boy racers.
"They'll bring it in, try and get a warrant, [it] fails miserably, they'll take it back - basically put it back to standard and once it's passed a warrant they'll be back to their old tricks again."
MTA spokesman Ian Stronach said the national fleet age was old compared to other benchmark countries.
Australia's fleet is an average 10 years old, the United Kingdom's 7.4, America's 10.8 and Russia's 11.8.
As a consequence, New Zealand's fleet lacked standard modern safety features like airbags and ABS braking, Mr Stronach said.
The comments follow the Government's September announcement of a shake-up of the vehicle licensing system, including a proposal to extend the six-monthly WoF tests on cars more than six years old to 12 months.
Mr Stronach said economic factors were forcing people to keep old cars longer.
"Our slow-moving economy and our poor maintenance culture are in head-on collision."
While 13 years was the average age for Kiwi vehicles, Mr Stronach said many cars were much older.
There was nothing wrong with an older car which was well maintained, but only up to a point, Mr Stronach said.
He said it was dangerous to think just because a car was well maintained it would be as safe as a modern car.
AA spokesman Mark Stockdale said while older cars had less safety features, it did not make them less safe. Vehicle defects were a minor contributor to vehicle crashes, he said.
"Only about 2.5 per cent of all crashes are due to some sort of a vehicle defect and only 0.4 per cent where the vehicle defect was the sole cause of the crash. The reality is, very few crashes are caused by older, worn vehicles."
Average car fleet age
Masterton: 14 years
Carterton: 14.8 years
South Wairarapa: 14 years
Wellington: 10.9 years
Auckland: 11
New Zealand: 13.03
Australia: 10