Residents of dozens of NSW suburbs are being urged to monitor any possible coronavirus symptoms after viral fragments were detected at two wastewater plants.
It comes as states and territories on Saturday recorded another day of no new locally acquired cases.
Seven cases were recorded in hotel quarantine – three in Victoria, two in Western Australia and one each in NSW and the Northern Territory.
NSW Health said its sewage surveillance program had recently detected fragments of the virus that causes COVID-19 at the Liverpool and Glenfield plants in southwest Sydney.
The Liverpool plant takes in a catchment of almost 180,000 people, while the Glenfield plant has about 160,000 in its catchment.
Everyone living or working in the affected suburbs is urged to monitor for symptoms and isolate immediately and get tested if they appear.
An earlier detection of viral fragments at plants in Camellia and Auburn is likely to reflect known confirmed cases in the area, NSW Health said.
Victoria has reopened its border to almost all of NSW, which has now gone six straight days without a local virus case.
WA will follow suit from Monday but NSW travellers will still be required to self-isolate for two weeks upon arrival.
Victoria, meanwhile, is allowing more household visitors after the state government further lifted restrictions in time for the long weekend.
The indoor gathering limit at homes doubled at midnight on Friday from 15 people to 30.
The smaller limit on home gatherings had been introduced on New Year’s Eve, as the state recorded a cluster of new cases linked to a Black Rock Thai restaurant.
Victoria has now gone 17 days with no community cases.
State and federal leaders at Friday’s national cabinet meeting resolved to keep halved international arrival caps in place until February 15.
Prime Minister Scott Morrison is adamant the cap is needed to protect public health and ease pressure on quarantine providers.
There are 38,000 Australians seeking to return home and the list is steadily growing as the virus tears through large parts of the world.
Mr Morrison said the government had exceeded its own expectations, with 79,000 people returning to the country since September.