Experts are renewing calls for people to get their Covid booster in the wake of rising infections and hospitalisations.

Chief Medical Officer Paul Kelly said there were continuing reinfections from the Omicron variant that were escaping immune responses bolstered by vaccinations.

"We are seeing reinfection, we know that Omicron, having had a previous dose of one of the other variants of Covid, did escape that immune protection from both a previous dose and from only two doses of vaccine," Professor Kelly said.

"Sub-variants of Omicron BA.4 and BA.5 have now become the dominant strain in the UK (and are) growing particularly on the east coast of Australia."

But while the variants are more transmissible, there has not been any uptick in the severity of the disease.

"We will see reinfections over the coming weeks and months, unfortunately with that one, but what we're not seeing is a large increase in severe disease," Prof Kelly said.

"That's really due to the vaccine protection which can be boosted with a third and fourth dose."

Treatments and the availability of Covid antivirals have also aided in the response to serious cases of the virus, Prof Kelly said. 

Epidemiologists are also warning of a rise in reinfection and people developing severe bouts of the disease.

Deakin University epidemiology chairwoman Catherine Bennett said the broader community would be at greater risk of infection in weeks to come.

"We are seeing what we feared might happen, particularly the convergence of a troubling flu season after having had a couple of years of respite," she said.

NSW health authorities said Omicron variants were likely to become the dominant virus strains in coming weeks, warning of a rise in infections even among those who have already had Covid.

Professor Bennett said each new variant would be more transmissible, making it more difficult for people to avoid catching the virus when out in public.

She said some people who received their booster early in the rollout may see their protection start to wane.

Only 70 per cent of the eligible population have received their booster.

"The message is, if you haven't had your booster, have it," Prof Bennett said.