* South Australia joins Victoria and Sydney in lockdown

* New cases ease slightly in New South Wales, Victoria

* 21 NSW cases spent time in community while infectious

SYDNEY, JULY 20

More than half of Australia's 25 million people were under lockdown on Tuesday after a third state adopted movement curbs to rein in the highly contagious Delta variant of coronavirus.

Australia's infections and deaths are well below other developed nations, but its use of lockdowns, prompted by a sluggish vaccination campaign, is putting pressure on the national government, with polls at their lowest in a year and just months before elections are due to be held.

South Australia, a state of 1.8 million, imposed a seven-day lockdown after detecting five infections linked to a returned traveller, just as the neighbouring state of Victoria extended by a week a five-day lockdown that had failed to stop new cases.

'We hate putting these restrictions in place but we believe we have one chance to get this right,' South Australia premier Steven Marshall told reporters.

The largest city of Sydney, where the latest Delta outbreak started before spreading elsewhere, is in the fourth week of a five-week lockdown. Three regional centres were added to areas 250 km (155 miles) inland after a positive test there.

Sydney is the capital of New South Wales, the state that recorded 78 new cases on Tuesday, down from 98 a day earlier, for its biggest daily dip since Sydney went into lockdown.

At least 21 of the new cases were infectious in the community before being diagnosed. Authorities have said that figure should be near zero if Sydney's lockdown is to be lifted by a target date of July 30.

'We are seeing more hospitalisations, more admissions to ICU, more people on ventilators - we have to stop the spread of COVID,' Kerry Chant, the state's chief health officer, said in Sydney, referring to intensive care units.

Ninety-five people with COVID-19 are in hospital in the state, 27 of them in intensive care and 11 on ventilators. The state's five deaths in the latest outbreak take the national toll to 915, with a tally of just over 32,000 infections.

INFECTIONS ONLY A FRACTION

Australia has relied on lockdowns, tough social distancing rules and swift contact tracing to hold infections to a fraction of levels elsewhere, such as Britain and the United States.

However health services are being tested by the virulent Delta variant and low vaccine coverage in Australia, where just over 14% of adults are fully vaccinated.

Although 13 million Australians were under lockdown, the country's health minister defended its pandemic response as having saved thousands of lives.

'The scope and scale between the rest of the world and Australia are immeasurably different and we shouldn't lose sight of what has been achieved on an extraordinary level,' Greg Hunt told reporters.

A five-day snap lockdown in Victoria set to end on Tuesday night was extended by a week until July 27, as officials sought more time to quell the outbreak.

'There are chains of transmission not yet contained that we don't know about,' state premier Daniel Andrews said in Melbourne.

'The speed with which this has moved through the Victorian community confirms that we did the right thing to lock down, and it also sadly confirms that we need more time.'

Victoria's nine locally acquired cases, down from 13 a day earlier, followed a downward trend, but took its tally of infections to more than 80. All but one of the new cases were linked to the current outbreak, officials said.

The number of virus-exposed sites in Victoria has exceeded 300 since the first cases detected a week ago, linked to a team of infectious furniture movers from Sydney.