Hunter New England Health says three towns in the region have state-high levels of coronavirus in their sewage.
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Public health controller Dr David Durrheim said on Sunday that he was concerned about Raymond Terrace, Williamtown and Medowie, where the level of viral detection in sewage was "as high as anywhere we've seen in NSW".
He urged anyone with "even the mildest of symptoms" to get tested and isolate immediately.
Dr Durrheim was also worried that three of the Hunter's five unlinked COVID-19 cases were in the Maitland area
He said the town had "high-risk transmission sites" at the Guzman y Gomez restaurant at Green Hills shopping centre, Maitland Christian School and Sudania cafe in High Street.
The Guzman y Gomez store is listed as an exposure site from 2.30pm to 8.30pm on August 7, 8 and 9 and from 5pm to 8.30pm on August 11.
The Hunter reported nine new cases on Sunday, a result NSW Premier Gladys Berejiklian described as "positive" and a "stabilisation".
Chief health officer Dr Kerry Chant also described the "settling" of cases in the region as pleasing, but health authorities remained concerned about the Maitland area.
Seven cases were reported in Maitland on Saturday and there were a further two among the nine new cases.
Three of the Sunday's cases are in Newcastle, three in Port Stephens, two in Maitland and one in Lake Macquarie.
Dr Chant has urged Hunter residents to remain vigilant about published exposure sites in the area.
NSW recorded 415 new cases in the 24 hours to 8pm on Saturday.
Another four people have died of COVID-19, including an unvaccinated woman in her 50s in western Sydney, a woman in her 70s in Royal North Shore Hospital who had only recently had her first dose of a vaccine, an unvaccinated man in his 80s at Liverpool Hospital, and a woman at Royal Prince Alfred who had been vaccinated but had underlying chronic conditions.