Authorities may start testing backyard mulch for bonded asbestos after the hazardous material cropped up in more transport projects across Sydney.
Testing along landscaped areas of the Prospect Highway in the city's west and three sites along the City and Southwest Metro line returned positive samples, Transport for NSW said on Wednesday.
Recycled mulch containing bonded asbestos has already forced the closure of several parts of Rozelle Parklands in the city's inner west as well as other areas associated with the road interchange underneath the site.
The latest discoveries prompted the Environmental Protection Authority to halt the parklands' mulch supplier, Greenlife Resource Recovery, from further distributing its product.
"This mulch cannot be supplied to anyone unless and until the investigation of the EPA concludes that it is safe," EPA acting chief executive Nancy Chang told reporters.
Ms Chang welcomed inquiries to a new public hotline from anyone concerned about recycled mulch at their home or business.
If need be, inspections and samples would be taken, she said.
But she was unwilling to rule in or out any potential source of the contamination while all possibilities were being probed.
Those included the bonded asbestos entering the mulch after being distributed by Greenlife or being on site before the mulch was laid.
Many of the sites where asbestos had been found had a history of asbestos contamination, the authority said.
Greenlife on Wednesday firmly rejected any blame, saying independent expert tests of its mulch had shown unequivocally that there was no asbestos present.
"We are of course extremely concerned that members of the public, including children, have been exposed to asbestos at the Rozelle Interchange and Parklands," the company said on Wednesday.
"But, as detailed, we categorically refute this contamination was from our delivered mulch."
Transport Secretary Josh Murray would not rule out further positive samples at the sites of recent transport projects.
All landscaped area where recycled mulch has been used in the past year were being tested, he said, including those associated with the Metro line to the under-construction Western Sydney Airport.
"We do believe any exposure across additional sites is limited, however we're not prepared to rule anything out," he said.
The positive results reported on Wednesday came after testing at the Prospect Highway project between Prospect and Blacktown in the city's west.
The sites are fenced off to prevent any public access and a remediation plan is being developed.
Targeted testing at five Sydney Metro areas inaccessible to the public had also uncovered the material in Dulwich Hill, Canterbury and Campsie.
Meanwhile, Mr Murray said the Rozelle Parklands were now expected to remain closed until April with work to "suck" up and then replace 10 tonnes of mulch to take months.
The parklands opened in mid-December only for a child to uncover asbestos in mulch adjacent to the playground a little over a fortnight later.
It took until January 10 for park operator Transport for NSW to confirm the presence of asbestos and begin fencing off the area with more parts closed as extra tainted mulch was identified.
That delay irked Transport Minister Jo Haylen, who said an email with asbestos in the title should get an immediate response, regardless of the time of year.
She also wanted testing regime that failed to identify asbestos to be examined.
But the priority for now remained public safety and remediating affected sites, she said.
While low risk compared to friable asbestos, bonded asbestos can weaken and have the potential to turn into powder as it ages and is exposed to the elements.
