Owners of the country's 217,000 diesel cars, as well as truck and bus operators, are being urged to ensure regular maintenance after confirmation that their exhausts can cause cancer.
The exhaust from diesel was added to the World Health Organisation's list of most carcinogenic substances yesterday. It ranks alongside arsenic, asbestos, formaldehyde, mustard gas and plutonium as a major health hazard.
Although its emissions were previously classed as probably carcinogenic, agency working group chairman Christopher Portier said definitely yesterday that "diesel engine exhaust causes lung cancer in humans". The agency said there was also a "positive association" between diesel exhaust and a greater risk of bladder cancer.
It noted that large numbers of people were exposed to diesel fumes in everyday life, at work and elsewhere.
It reaffirmed an assessment of petrol exhaust as "possibly carcinogenic to humans".
Another agency spokesman, Kurt Strait, said it had learned from other carcinogenic substances that initial studies showing a risk in heavily-exposed occupational groups were followed by findings of danger to the general population as well.
Agency director Christopher Wild said the latest finding sent "a strong signal that public health action is warranted".
A Ministry of Transport spokeswoman said last night that the global health warning would be considered as part of "ongoing work on vehicle emissions" in New Zealand.
She said the Government had already put in place "the world's most stringent requirements" for vehicles entering this country's fleet through a rule which prohibits importing diesels made in Japan before 2005.
A draft amendment, flagged in April by Transport Minister Gerry Brownlee to ensure New Zealand continued to import vehicles built to the highest exhaust standards, would be put up for public consultation this month for implementation by the end of the year.
Sulphur in diesel - a major generator of tiny cancer-causing particles lodging in the walls of the human lung - has been reduced to 10 parts per million from 3000 in 2001.
But a report in March by Auckland Council air quality scientists warned that cleaner fuel and better-maintained vehicles are not enough to reduce transport emissions, which are estimated to cause more than 250 premature deaths a year through the region.
Automobile Association spokesman Mark Stockdale said that although this country's fuel standards had become world-class, the WHO report confirmed a grave responsibility for drivers of diesel vehicles to have them regularly maintained.
Although these were more energy-efficient than petrol counterparts, they emitted a wider range of pollutants of greater risk to human health.
He said the danger lay in a large number of drivers switching from petrol to diesel cars, many not knowing about extra servicing requirements.
These included a need to replace fuel filters at least once a year, and possibly up to four times a year.
"Vehicle dealers need to explain to people who want diesels because of their reduced fuel costs that some of those savings will be offset by a higher cost of servicing that the owner has a social, moral and legal obligation to do," Mr Stockdale said.
Diesel in same category as:
* Mustard gas