South Australia will head into lockdown for seven days from 6pm on Tuesday after it recorded a fifth case of Covid-19.

Premier Steven Marshall announced today that health authorities had confirmed the highly-contagious Delta variant had infiltrated the state.

Marshall said the bad news had left the government with no other alternative but to impose a hard lockdown.

"We hate to put these restrictions in place but we have just one chance, one chance to get this right," he said. "If we don't get this right we could be in an extended lockdown situation and that is something we have always wanted to avoid."

For residents in South Australia there will be only five reasons to leave the home:

– Care and compassionate grounds
– Essential work
– Purchase of essential goods, such as groceries
– Medical reasons including vaccination and testing
– Exercise, but limited to family/household groups

Marshall said although he appreciated the inconvenience for regional South Australians the entire state needed to be locked down until authorities could get a clear picture of the virus' spread.

He said the alternative would be "catastrophic" for the state.

"What we've seen in Victoria for example is a very rapid movement out of Greater Melbourne," Marshall said. "This current variant is very concerning so until we have much clearer idea of how widespread this is, we have to take every single precaution to keep our state safe."

Marshall earlier revealed on morning radio the fourth case, a man in his 60s, was a close contact of the man in his 80s who tested positive on Monday.

An 81-year-old man and two of his close contacts – his daughter and a man, both aged in their 50s – had previously tested positive for the virus.

The fifth case is a man in his 60s who picked up the virus at a Greek restaurant on Halifax St, Adelaide on Saturday.

"This fifth case is far more worrying," Marshall said.

Chief Public Health Officer Nicola Spurrier said the fifth case had attended a birthday event at The Greek on Halifax, where another positive case was present.

Spurrier said health authorities were worried about the potential spread at that site as people often talk loudly and sing at events such as birthday parties.

"This is the time to stay put. This is the time not to move around," Spurrier said. "The virus doesn't have legs … if we stay put we will be able to get on top of it."

She said it was still to be confirmed if the Delta variant was linked to the NSW outbreak.