SYDNEY - Health officials in Australia's New South Wales state issued an alert on Saturday (Jan 23) for more than 300,000 people after traces of the virus that causes Covid-19 were detected in sewage in Sydney's west.

This is despite the state recording no new locally acquired cases for the sixth consecutive day on Saturday.

Viral fragments were recently found at treatment plants in Liverpool and Glenfield, NSW Health said in a statement on Twitter.

The two plants take in catchments of about 340,000 people.

The latest findings under NSW Health's sewage surveillance program follow detections of the virus at treatment plants in Camellia and Auburn on Friday (Jan 22), the Sydney Morning Herald reported.

"While this likely reflects known recent confirmed cases in these areas, NSW Health urges everyone living or working [in these areas] to monitor for symptoms and get tested and isolate immediately if they appear," NSW Health said on Saturday.

The findings are part of the NSW government's wider testing of sewage across the state for virus fragments.

According to NSW Health, the NSW Sewage Surveillance Program tests untreated sewage for Covid-19 virus fragments at more than 60 treatment plants across the state.

The testing provides an early warning of an increase in infections in an area, and potentially gives an estimate of undetected infections in the community, the health authority says.

On Saturday, Australia logged a sixth straight day of zero coronavirus cases in the community nationwide. It has recorded 28,700 cases in total, the overwhelming majority in Victoria, and 909 deaths.

Victoria recorded its 17th straight day without any new local coronavirus cases on Saturday.