A supply-chain investigation has turned up 75 asbestos-contaminated Sydney sites but more may yet be revealed from a handful of pending tests for tainted mulch.
The tracing operation has been completed, with samples returning a series of positive results in mulch at city parks, schools, hospitals and churches.
It was sparked when fragments of bonded asbestos were detected in mulch at recently-opened inner city Rozelle Parklands in January.
The discovery of tainted mulch at Liverpool West Public School followed in February.
That school will reopen on Monday after being cleared by an independent hygienist on Thursday.
"The decision to temporarily close the school site was not taken lightly and we appreciate the community's patience while we managed the situation as safely and as quickly as possible," Education Minister Prue Car said on Thursday.
Classes continued at nearby Gulyangarri Public School as remediation works were done.
The state's Environment Protection Authority (EPA) has been working for eight weeks to determine the source of the tainted mulch and how it entered the supply chain.
About 6500 tonnes of mulch have been tracked, assessed and tested and nearly 1200 samples taken for analysis.
All sites that returned positive results had small levels of contamination and NSW Health advised the risk to the public was low.
The EPA's criminal investigation into the saga continues.
Environment Minister Penny Sharpe said while the initial phase of testing was complete, a handful of results were yet to come back.
"It's possible there might be a handful (more sites) ... but we've gotten to the bottom of what should be tested," she told parliament on Thursday.
The NSW government has pledged to tighten regulations and strengthen penalties for a range of environmental crimes.
Ms Sharpe said the initial discovery of asbestos in mulch had led to the biggest investigation in the EPA's history.
"Asbestos is illegal in any product and should not be there," she said in an earlier statement.
"While the EPA works on its criminal investigation, the NSW government will move to tighten regulations and make further changes as required."
Premier Chris Minns said a "polluter-pays regime" remained in place and he would wait until the investigation was over before giving a figure on remediation costs or the potential bill for taxpayers.
Many of the contaminated mulch supplies have been confirmed as linked to Sydney's Greenlife Resource Recovery.
Greenlife is challenging an EPA prevention order and says it is confident the mulch left its facility free of contamination.
The Land and Environment Court will hear directions on the challenge on Friday.
City of Sydney mayor Clover Moore said the scandal was a major failure of regulations.
"We should be able to trust that the mulch we're contracted to receive has gone through all the proper processes and is safe," she told ABC Radio.
The Sydney outbreak triggered inspections in Queensland with asbestos detected in compost at waste facility NuGrow's Ipswich site.
Up to 25 sites were initially identified as having received potentially tainted soil from NuGrow.
Authorities on Thursday said 90 sites in Queensland had been identified, with more than 4000 tonnes of soil leaving NuGrow.
Results are still pending for a number of sites, with no further positive tests to date.
