South Australia will move into a full seven-day lockdown on Tuesday afternoon after a cluster of COVID-19 cases grew to five.
The fifth case is someone who dined at a restaurant in Adelaide where previously infected people were also present on Saturday night.
The lockdown rules will come into effect from 6pm with South Australians only allowed out for five reasons.
Mr Marshall said it had also been confirmed the current cases involved the Delta variant of the disease.
“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.”
South Australians will only be allowed out of their homes for essential care, essential work, the purchase of foods and essential goods, for medical reasons and for limited exercise.
The SA cluster began with an 81-year-old man who entered the country from Argentina, his daughter and another two men.
As the number of cases and exposure sites continued to grow, the premier said more testing sites would open in Adelaide.
The outbreak has plunged thousands of people into quarantine with the list of more than a dozen exposure sites including at least four major shopping centres in Adelaide’s north and northeastern suburbs.
That list was expected to grow further on Tuesday.
The elderly man had returned to Australia via NSW, where he spent 14 days in quarantine.
He returned to SA on July 8 and presented to a suburban hospital over the weekend after developing symptoms.
Business SA Chief Executive Martin Haese said the new restrictions were another blow to the already struggling hospitality, events, tourism and retail sectors.
“We hope there will be no extension to the restrictions beyond Friday.”