The Hunter's COVID-19 medical controller said critical care services would remain available in the "stretched" system despite NSW Health declaring a "red alert" on Thursday night.

The alert, which was declared due to increased transmission and the emergence of Omicron, is to protect vulnerable patients and exhausted health workers, who now face a busy Christmas at a time they were hoping to get a rest before cases exploded in the region.

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It means visitors are temporarily banned from NSW healthcare facilities except for special circumstances such as essential patient needs and compassionate reasons. It's a decision the health service said was "not made lightly".

Staff at HNE Kids Health's community services have also told The Herald they were advised that unless they have received a text message from NSW Health confirming they are a close contact, they are expected to show up to work in face-to-face roles.

Hunter New England Health medical controller for COVID-19 Dr Paul Craven said hospitals "from Maitland down" were struggling with staff after employees were swept into isolation from recent exposures.

He said there were 140 people furloughed on Wednesday at John Hunter Hospital alone and described the system as "stretched".

"We're trying to get people back as quickly as possible," he said.

But Dr Craven said some staff had returned, and stressed that all critical services were still operational.

"Our hospitals are amazing, we work as a system," he said. "We do pull staff from other areas."

With COVID circulating widely, there is also a risk of increased hospitalisation of positive patients.

Dr Craven said the system had been in "brace position" for an influx for two years now, but had noticed as infectiousness had increased, people were not getting as sick, particularly due to high vaccination rates and quality medication.

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