South Australia has reported cases of the BA.4 and BA.5 Omicron strain in international travellers while Victoria detected the BA.2.12.1 variant in a wastewater catchment.

The BA.4 and BA.5 strains have not previously been reported in Australia.

South Australia's Chief Public Health Officer Professor Nicola Spurrier said health authorities are monitoring the transmissibility and severity of the new variants "very closely".

"The expectation is this will be quite similar to the other Omicron variants," Professor Spurrier said.

"I'm sure we'll start to see those sorts of cases picked up in other states."

Infectious diseases expert Associate Professor Stuart Turville said more time is needed to understand the transmissibility and severity of these variants.

"BA.4 and BA.5 have a small change in a part of the virus that in previous variants helped transmission and also enabled the variant to evade antibodies modestly," Professor Turville said.

"The key thing is that these are small changes in the virus, and not unlike what we have seen before.

"They may not resemble the large shift we saw with Delta to Omicron."

Concerns over transmissibility of BA.2.12.1

Early evidence suggests the BA.2.12.1 strain is more transmissible than BA.2 but does not cause more severe disease.

Professor Sanjaya Senanayake told Today said the new Omicron sub-variant is currently "dominating the US landscape".

"People don't seem to be getting any sicker than with earlier Omicron variants, but it does seem to be more transmissible."

At the beginning of the pandemic, there were concerns about the capacity of intensive care units and the availability of ventilators in hospitals.

"(This) means that resources, doctors, nurses and ward space will have to continue to be dedicated to that," he said.

"That's going to be the real struggle here."