Western NSW Local Health District Chief Executive Scott McLachlan said this sample was in addition to the confirmed case in Molong which health authorities have been managing.

The test of the sewerage system was undertaken on Monday with the fragment detected in the past 24 hours, Mr McLachlan said on Wednesday.

"As a result of that we're extra cautious. This is time for us to be aware that COVID could be creeping around in Molong or around our region so I would really encourage people to come out and get tested," he said.

It was not known if the fragment had come from someone who resided in Molong or had just travelled through.

Health authorities were certain it had not come from the one confirmed case as that person was not linked to the town's sewage infrastructure.

It could not be determined if the person the COVID fragment came from still had the virus due to the fact it can stay in a person's system for up to a month after they have been infected.

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In the past week over 1100 people in Molong have been tested - around half of the community - which Mr McLachlan said was "fantastic to see". None of these tests have returned positive results.

News of the COVID-19 fragment in Molong came just after the NSW premier's announcement that the government would be redirecting doses of the Pfizer vaccine from regional and rural NSW to vaccinate Year 12 students in the worst-affected Sydney LGAs.

The WNLHD was still awaiting the state government's advice as to whether this will mean Pfizer doses already in the region will be similarly sent to Sydney.

Mr McLachlan said there was still a "good supply" of the AstraZeneca vaccine available in the Western NSW region and encouraged residents to check their eligibility for vaccination.

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"We know it's getting to a really serious situation [in Sydney]. The intention is to try and get kids back to school, to get workplaces resuming, [and] life back to normal as quickly as possible and so I'm very supportive of the vaccination programs in Sydney," he said.

"If it does mean that we need to constrain some of the supply of vaccines in Western New South Wales to help out then I think that's the right thing to do.

"What we do know is that there's a lot of vaccine available throughout the region at the moment for people to access, particularly through their GPs."

The Greater Sydney lockdown and regional restrictions will continue for at least another four weeks after the state recorded 177 new cases of COVID-19 overnight.

NSW premier Gladys Berejiklian said that of the new locally-acquired cases, 46 had been in the community while infectious.

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