Railway workers have been exposed to potentially hazardous asbestos after the deadly dust was found in locomotives brought in from China.

The breach of a 10-year ban on the import of products containing the carcinogenic fibre is not the first incident of its kind.

Unions are now demanding tougher policing of Chinese imports, describing the current asbestos-free certificates as a farce.

Last year freight carrier SCT imported 10 locomotives made by China Southern Rail (CSR) to tow iron ore bound for China to port.

To comply with the decade-old Australian ban on asbestos imports, they were certified asbestos-free. However, this was not the case.

National secretary of the Rail, Tram and Bus Union Bob Nanva says maintenance workers raised concerns about the dust.

"We had our maintenance workers repairing a number of diesel engines," he said.

"They identified a lot of white dust among those engines and asked the question as to whether or not that dust was safe."

The workers' concerns were justified. White asbestos - or chrysotile - was found throughout the locomotives, in insulation around the exhaust and muffler system, around coolant pipes and in the brake exhaust section near the roof of the driver's cabin.

Workers reassured despite dangers

Mr Nanva said workers were initially told there was nothing to worry about.

"They were assured on numerous occasions that there was nothing to be alarmed about, but on subsequent testing of that dust they have identified asbestos," he said.

Last month, at a cost of more than $1 million, the locomotives were pulled from service.

Most were quarantined at SCT's centre at Penfield in northern Adelaide, where professional asbestos removalists in protective suits and masks have been stripping the asbestos out of the trains.

The company's subsidiary Specialised Bulk Rail says its first priority has been the safety of its staff, some of whom it concedes may have been at "some risk" when the asbestos-containing insulation blankets were "damaged or ripped".

Mr Nanva says the workers would have been regularly at risk.

"These are maintenance workers that repair these trains day in, day out, and would have been exposed to these fibres day in, day out," he said.

Chief executive of the Asbestos Safety & Eradication Agency Peter Tighe says it shows certifications from China are questionable.

"It's another example, I think, of the lack of compliance in relation to certification from Asia, and more specifically China, that asbestos-free certification is really questionable out of those areas," he said.

Asbestos ban broken before

This is not the first time China has broken the Australian ban on asbestos.

Last year more than 25,000 Chinese-made Great Wall, Chery and Geely cars were recalled after asbestos was discovered in their engine gaskets and brakes.

In decades to come experts expect hundreds of thousands of Chinese casualties from asbestos.

A 1980s film by Szechuan University smuggled out from China shows the tragic story of China's own Wittenoom - at Dayao, in the province of Yunnan - where asbestos exposures had led to the fatal cancer - mesothelioma.

Back in Australia, it was the same type of blue asbestos, from the Wittenoom mine, that lined Melbourne's blue Harris trains, potentially poisoning passengers when the walls were broken.

So dangerous were the trains they were sealed in plastic and buried in quicksand at a quarry in Clayton.

Blue asbestos, which is more likely to cause the cancer mesothelioma, is now banned in both countries - but China is now the world's largest user of white asbestos, which Perth's asbestos expert Professor Bill Musk warns still causes cancer.

"The risk of lung cancer from white asbestos may be more than from blue asbestos given the same amount of exposure," he said.

Growing cancer epidemic in China

Much of China's white asbestos has been mined near Mongolia by prison labour.

Conditions there and in Chinese factories are extremely dusty and long-term studies of asbestos workers have revealed a growing cancer epidemic.

Mr Navna says asbestos is a "ticking time bomb".

"The fact that you have family station wagons, trains, numerous components from China being imported into Australia without the requisite checks is a grave concern to us," he said.

"It should be a great concern to Australian consumers."

The giant state-owned China Southern Rail, exhibiting at this week's AusRail conference in Sydney, said in a statement that asbestos was clearly excluded from the specifications for the locomotives. It blames a sub-contractor for supplying the asbestos and insisted it will not happen again.

But, CSR's assistant general manager Li Huling said: "Although there was an explicit restriction in the use of asbestos, the interpretation of the definition of asbestos by our sub-contractor did not include chysotile [white asbestos] - as it was widely used in the world."

Several cases of Chinese companies breaking asbestos ban

White asbestos use in Asia is expanding, and China is not the only country to break the Australian ban.

Recently asbestos was discovered in engine gaskets of two tugboats imported from Singapore in 2008 for use in the port of Fremantle.

Australian Institute of Marine and Power Engineers assistant federal secretary Martin Byrne says two tugboats that came to Australia had asbestos-free certificates.

"There were declarations by the shipyard that built the vessels that there was no asbestos-containing material at all in those vessels," he said.

"When we started to work them and needed to repair them and started to have to take them apart, it was discovered that there were asbestos-containing materials."

Like the train drivers' union, Mr Byrne says the marine engineers institute has bitter experience from past exposures to asbestos of its tragic consequences.

"We get the phone calls. We have the members coming to us after they had the diagnosis from the doctor of mesothelioma," he said.

"I know, personally, deep inside me, that as soon as the guy tells me that it's a death sentence."

Mr Nanva says he is now wondering whether Sydney's new fleet of Waratah passenger trains - part of which were sourced from China - might also contain asbestos, something Transport for NSW says it is satisfied is not the case.

"We have no confidence that any component or train that is manufactured in China and imported into Australia is free of asbestos," Mr Nanva said.