South Australia will spend the next seven days in lockdown as authorities try desperately to contain an outbreak of the fast-moving Delta coronavirus variant.
Five cases had been reported in the emerging cluster by Tuesday afternoon, with officials declaring the next 24 to 48 hours crucial to prevent virus infections running wild.
The fifth case, a man in his 60s, dined at a restaurant in Adelaide where previously infected people were also present on Saturday night.
They had all attended a birthday party in a private function room which authorities said had the potential to act as a superspreader event.
During the week-long lockdown beginning 6pm on Tuesday, South Australians will only be allowed out for five reasons - to provide essential care, to seek medical assistance, to buy essential food and other goods, for essential work, or to exercise for up to 2.5 hours.
Premier Steven Marshall said it was vital for SA to "go hard and go early" to have any chance of containing the outbreak.
"We hate putting these restrictions in place but we have one chance to get this right," the premier said.
"We are moving as quickly as we can to slow and stop the spread of this cluster.
"We know these restrictions will take a heavy toll. But we've always had a situation where we go hard and we go early in South Australia.
"We've got to stop movement around this state and we've got to stop it immediately."
The lockdown has raised concerns for workers who could lose pay with the federal government late on Tuesday declaring metropolitan Adelaide a coronavirus hotspot to allow for payments of up to $600 for people who lose work this week.
"South Australian should go into this week knowing that the state is acting early and decisively to get on top of this outbreak," Federal Finance Minister Simon Birmingham said.
"This is a really tough time for so many South Australians, but none more so than those are who are losing casual shifts, losing part-time jobs and are going to feel some financial stress."
Business SA Chief Executive Martin Haese said business owners were shattered but had been assured help was on the way.
"These restrictions will hurt, and businesses will lose money, but we are resilient and will get through this," Mr Haese said.
"This rapidly evolving situation provides a small amount of comfort that we have got this early, giving us every reason to believe this lockdown will be over quickly."
As the number of cases and exposure sites continued to grow, authorities said more testing facilities would open with the first of those a drive-through facility at Ridgehaven, in Adelaide's northeast.
The outbreak has already plunged thousands of people into quarantine, with more expected to be forced into 14 days of isolation.
Chief Public Health Officer Nicola Spurrier urged everyone, especially those in quarantine, to stay home.
"The virus doesn't have legs, it moves around when people move around," she said.
"So if people stay put we will be able to get on top of it."
Professor Spurrier said contact tracing was underway in regard to the fifth positive case which was confirmed early on Tuesday morning.
She said the situation was currently more dangerous than when SA had an outbreak of cases known as the Parafield cluster in November last year.
The new SA cluster began with an 81-year-old man, who entered the country from Argentina, his daughter and another two men.
He had returned to Australia via NSW, where he spent 14 days in quarantine.
He arrived back in SA on July 8 and presented to a suburban hospital over the weekend after developing symptoms.
All the confirmed cases are known to the man or his extended family.
Australian Associated Press