A spike in COVID-19 infections seen in Australia's two most populous states over the Christmas and New Year period may have reached a plateau, data suggests.

COVID variant JN.1 drove a wave of infections responsible for a steep increase in hospitalisations across NSW and Victoria over the holiday period.

Testing by NSW Health showed the proportion of JN.1 cases rising sharply from late November and increasing into December.

But the latest sewage data indicates the transmission may be plateauing, though activity remains high.

A NSW Health COVID-19 weekly data overview report showed the virus activity has decreased in the week ending January 20.

Emergency department presentations in all age groups declined, though the the report cautioned the figures may be influenced by primary healthcare availability over the holiday period.

"Sewage data indicates that transmission may be plateauing in the community in the Sydney catchment," it said.

COVID-19 PCR test positivity was recorded at 13.9 per cent, a decline on the 17.9 per cent recorded the week ending January 6.

Health Minister Ryan Park said while it is encouraging to see a COVID plateau, the best protection remains vaccination.

"Now is not the time to become complacent," he said.

"If you're experiencing cold or flu symptoms, stay at home. If you need to go out, mask up.

"If you haven't had your 2023 booster, go get one."