Hundreds of sites across Sydney are potentially contaminated with asbestos, the NSW premier warns, as the government scrambles to identify the extent of the exposure.

Mulch laden with asbestos has already been detected in at least 22 sites after contaminated material was first found at the Rozelle Parklands in Sydney’s inner-west in January.

“I’m sorry to say but the truth of the matter is that the number of [contaminated] properties would be very large right across Sydney,” Premier Chris Minns said on Thursday.

Asked if the likely figure for the number of contaminated sites was in the hundreds, Minns said: “Yes.”

Contact-tracers working to find potential sites containing the contaminated mulch have had numbers boosted to speed up the process and reduce exposure to the public.

But Minns said resourcing issues prevented the government from closing all parks before they were tested.

“Not every projected park is testing positive to asbestos, but to lock every single park or school or hospital up would be beyond our resources right now,” he said.

The latest discovery at a primary school in Liverpool has resulted in hundreds of students learning from home before being shifted to another public school nearby while the mulch is removed.

Education Minister Prue Car said the clean-up would take longer than expected because the mulch had been incorrectly placed.

“We’ve obviously got a lot of a serious investigating to do about what happened at the actual construction of this [playground] upgrade,” she earlier told Nine’s Today program.

“There is supposed to be soil underneath the mulch in the garden bed, but instead of doing that the contractor in question has in fact just put metres of mulch.”

Car said the contractor involved was being investigated by the NSW Environment Protection Authority.

“It is a seriously unlawful act to contaminate mulch with something like asbestos,” she said.

Opposition environment spokeswoman Kellie Sloane has called for a central register of all sites under investigation by the state’s environmental watchdog to be made available to the public.

She said the register should inform the public in real time and give them the assurance to go about their daily lives.

The EPA issued landscaping supply company, Greenlife Resource Recovery, with a prevention notice after the watchdog determined it supplied the mulch used at Rozelle.

The supplier has been linked to all 22 sites across Sydney found with asbestos-tainted mulch.

The company said its testing showed mulch stockpiled at its facility was free of asbestos contamination and it was confident the material was also clean when delivered to contractors for landscaping.

The EPA probe has grown to involve 120 investigators, who are working to trace the supply of mulch.

NSW EPA chief executive Tony Chappel said the whole supply chain was under scrutiny.

– AAP