SYDNEY: AUSTRALIAN authorities on Monday warned the slow rollout of COVID-19 vaccine booster shots could unleash a new wave of infections amid the threat from the highly contagious BA.2 sub-variant of the Omicron coronavirus strain.

Australia battled record cases and hospitalisation rates during the initial Omicron wave, but they have steadied over the past six weeks. Most states have been easing social distancing rules, with mask requirements being rolled back at indoor venues and businesses asking staff to return to offices.

But daily infections could likely double in the next four to six weeks as the new sub-variant looks set to become the dominant strain, New South Wales Health Minister Brad Hazzard told broadcaster ABC on Monday, leaving "more people in hospital and more people possibly to pass away sadly."

Around 20,000 new cases were reported in Australia by midday on Monday with two states due to report later, while four deaths were registered. More than 3.1 million cases and 5,590 deaths have been recorded since the pandemic began.

According to official data, just over 57% of people above the age of 16 have received a third dose of the COVID-19 vaccine in New South Wales, home to a third of Australia's 25 million people, trailing the national average of 65%. Around 95% have received two doses.

"There is a degree of confusion," Hazzard said, admitting there was "a big problem" with people coming forward to get their boosters. More than two million people in the state - with a total population of eight million - are currently eligible for their booster dose but have still to get the shot.

The World Health Organization, based on initial data, said last month that the BA.2 variant appears to be more transmissible than the original BA.1 sub-variant.

Health experts and epidemiologists have asked authorities to consider reintroducing some restrictions, including making masks mandatory in supermarkets and other indoor venues.

But Prime Minister Scott Morrison over the weekend said the nation's political leaders want to move to a new phase of living with COVID-19 as though it were the common flu.