South Australia has imposed widespread coronavirus restrictions, including closing all non-essential retail outlets, following the discovery of a second COVID-19 case.

The daughter of an 81-year-old man has now tested positive along with her father after the pair arrived from Argentina some time ago.

Premier Steven Marshall says five other close contacts of the pair have tested negative so far but authorities are very concerned about the current situation.

“We only have one chance to get this right. Our focus from day one has been to go hard and to go early to minimise the level of disruption,” he said.

“We’ve got to respond quickly.”

New SA restrictions include a density change to one person to every four square metres in all venues, while all team and contact sport is to be cancelled.

All dining will need to be seated with a ban imposed on dancing and singing.

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Masks will be required in all public outlets, including shopping centres.

Fitness centres will be closed along with other non-essential retail outlets. Supermarkets and pharmacies and other essential businesses will continue to trade.

Schools will stay open.

The new rules will apply from midnight on Monday and will be reviewed on Friday.

Police Commissioner Grant Stevens said while SA was not moving to a full lockdown, the restrictions were significant.

“We know there are people who will struggle as a result of these changes. But the consequences of not taking action could be even more severe,” he said.

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“The intent here is to dramatically suppress all discretionary activity in the community.”

Authorities said earlier that the elderly man had returned to Australia through NSW, where he spent 14 days in quarantine.

He spent a further 10 days in hospital in NSW after suffering a fall before returning to SA on July 8, and presented to a suburban hospital over the weekend after developing symptoms.

The man had received one vaccine dose while in Argentina, but it was not known what type.

The man has been admitted to hospital, while his daughter has been taken to the Tom’s Court Hotel which is used to quarantine all positive COVID-19 cases.

“We don’t draw any conclusions as to where this infection came from. We simply don’t know and it is very early days,” Mr Marshall said.

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“Our primary concern at the moment is putting everyone who may have come into contact with this person into quarantine as soon as possible.”

Mr Marshall said the emergency department at Modbury Hospital remained closed while a risk assessment is performed, and would be progressively opened when it was safe to do so.

Some staff have been placed in quarantine.

The premier said the Elizabeth Vale Primary School would also reopen on Tuesday after being closed on Monday.

The school was closed as a precaution while checks were made to ensure the elderly man’s grandson, who is a teacher, didn’t attend over the two-week holiday period.

SA Health named five potential exposure sites, including an OPSM store at Golden Grove, a food warehouse at Hindmarsh, the Commonwealth Bank at Modbury and an Aldi Supermarket at St Agnes.

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There is also some concern for an outdoor reserve at Glenelg.

Chief Public Health Officer Nicola Spurrier said officials were assuming the man had the Delta variant of the disease, though that was yet to be confirmed.

She said it was known the Delta strain could “spread like wildfire”.

AAP