Asbestos-laden mulch installed at a new Sydney parkland required two rounds of testing before it was laid down, but the checks failed to detect the potentially hazardous material.
Thirteen contaminated samples have been found across the Rozelle Parklands since Wednesday after a child brought home asbestos from the inner-city site, prompting their carer to alert authorities.
Transport secretary Josh Murray said it would be safe to assume the contaminated mulch was in all landscaped areas of the park.
He also confirmed testing had been done as part of the initial development of the parklands, which are located above a major underground motorway interchange.
"We did have testing go through those processes and they are being looked at now by the (Environmental Protection Authority) to ensure that they were all robust, but they were delivered," he told ABC radio on Monday.
Mr Murray said he believed the testing was delivered "in line with what was required" at the time, noting NSW abided by a more stringent process than anywhere else in the country.
"Under the rules, as they've been explained to me, mulch is a product that is tested in its production," he said.
"So it's tested to be allowed to be sold under the process for delivering the parkland.
"The joint venture then receives the mulch and tests that again as it comes through for delivery."
Inner West Council Mayor Darcy Byrne has called for all the mulch to be replaced with organic, verifiably safe material.
"The whole batch of mulch that was used for the project was contaminated or is a contamination risk," he said.
"What's absolutely clear is this is not localised to one spot in the park, there's a problem with the entire batch."
Authorities are scrambling to devise a plan to rapidly remove the mulch in the 10-hectare park amid fears of wider contamination.
The NSW Environment Protection Authority on Friday afternoon said it was "extremely concerned" by the incident and had issued a draft order for park operator Transport for NSW to remove all contaminated material.
It also said it has visited the supplier and issued them with a formal notice to provide more information.
Targeted testing of the entire parklands has finished, but a number of samples from other sites outside the main park will be analysed in coming days.
The parklands have been closed since Wednesday after being open for less than a month.
