A lone woman, wearing a protective face mask, walks across a city centre bridge as the state of Victoria looks to curb the spread of a COVID-19 outbreak in Melbourne, Australia, July 16, 2021. REUTERS/Sandra Sanders

SYDNEY, July 20 (Reuters) - Australians admitted to hospitals from COVID-19 neared record levels on Wednesday as authorities urged businesses to let staff work from home and recommended people wear masks indoors and get booster shots urgently amid a major coronavirus outbreak.

Australia is in the grip of a third Omicron wave driven by the highly transmissible new subvariants, BA.4 and BA.5, with more than 300,000 cases recorded over the past seven days. Authorities say the actual numbers could be double that total, and Wednesday's 53,850 new cases was the highest daily tally in two months.

Prime Minister Anthony Albanese is resisting pressure to reinstate tough curbs to halt the spread of the virus, including making masks mandatory indoors, though he encouraged people to wear it.

"The truth is that if you have mandates, you've got to enforce them," Albanese told reporters on Wednesday. "Whilst there are mandates on public transport ... not everyone is wearing a mask."

Albanese said businesses and employees must decide together on any work-from-home arrangement, as unions called for employers to do more for their staff.

Employers must go beyond the government's pandemic leave payments and provide paid leave at full pay for workers who need to isolate, and offer free rapid antigen tests, Australian Council of Trade Unions President Michele O'Neil said.

"No worker should have to decide between putting food on the table or isolating with COVID," O'Neil said.

Last week, Australia reinstated support payments for casual workers who have to quarantine. read more

Australia's Chief Medical Officer Paul Kelly predicted the number of people ending up in hospital will soon hit a record high, and urged businesses to let more staff work from home.

Australia could see "millions" of new cases over the coming weeks, authorities have warned.

About 5,350 Australians are currently in hospital with COVID-19, not far off the record 5,390 recorded in January during the BA.1 outbreak, official data showed. Numbers in the states of Queensland, Tasmania and Western Australia are already at their highest since the pandemic began.

Many frontline health workers are also sick or in isolation, further straining the health system.

Australian doctors said masks must be made mandatory in indoor venues.

"We don't have optional seat belts, we don't have optional speed limits. There's a lot of limits on our freedoms that we accept because it's the right thing to do," Australian Medical Association President Omar Khorshid told radio station 2GB.

Authorities have also warned of a lag in people taking their booster shots.

So far, 95% of people above 16 have had two doses, helping keep Australia's total COVID-19 cases just under 9 million and deaths at 10,884, far lower than many countries. But only about 71% have received three or more doses.

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