Deadly loose-fill asbestos has been discovered in houses in Sydney's western and northern suburbs, the ABC has learned, the first positive tests for the carcinogen in the city.

Peter Dunphy from WorkCover NSW said investigators had unearthed four laboratory tests from the early 1980s that positively identified houses in Warringah, the Hills and Bankstown. A fourth test identified a house in Lithgow.

"The properties are in three areas, so it's in Yagoona, in the Bankstown local government area, they're at Carlingford in the Hills District local government area and and they're at Balgowlah in the Warringah local council area," Mr Dunphy said.

Mr Dunphy is leading an investigation into the number of homes across the state that could contain the deadly product, which was pumped into roof cavities during the 1960s and 1970s.

The authority expects more positive tests in the city, he said.

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"It will help us with the inquiry, to try and identify and quantify the number of properties in NSW," Mr Dunphy said of the finding.

"The records are quite useful in that they are definitive results".

Up to now, WorkCover had only found 17 houses in NSW containing the product - in Queanbeyan, Palarang, Yass and Orange.

The discovery has prompted the authority to add Warringah, the Hills and Bankstown to a list of local government areas eligible for free ceiling inspections. The free testing service is limited to homes built before 1980.

Mr Dunphy said his investigators discovered the test results while trawling archives of government agencies.

He said the records indicated the Sydney properties had the material removed from the ceiling, but there was a risk that residual fibres remained.

Mr Dunphy said it did not appear the loose-fill asbestos found in the Sydney properties was the notorious Mr Fluffy brand.

"It is interesting that each of the properties had different types of asbestos, so it will help us in trying to source where the materials were from, who was the supplier of the materials or whether they were homeowners who somehow sought the materials through other avenues," he said.

Mr Dunphy said the same records indicated there were less than 10 properties in Sydney that contained loose-fill asbestos.

"But we do want to make sure that we alert the public and that people are aware that there is that potential that we offer those free testing services," he said.

In the 1950s and 1960s, the home insulation business Mr Fluffy sold and installed loose-fill asbestos in ceilings of homes in the ACT and parts of NSW.

This year, more than 1,000 Canberra homeowners were told their properties were contaminated with the deadly carcinogen, despite safety assurances following a government-funded clean-up program in the 1980s.

In August, the NSW Government announced it would investigate the extent of the problem across the state.

Mr Dunphy said it was vital homeowners did not try to take samples themselves. Anyone wanting to arrange a free ceiling sample test should contact WorkCover on 13 10 50.

More information is available on the WorkCover web site.