South Australia has reported 18 new COVID-19 cases, its largest single-day tally for more than a year, with the outbreak forcing the governor and the opposition leader into isolation.
Sixteen of the new cases are linked to one event at suburban Norwood last weekend.
The other two involve travellers from interstate.
All cases who attended the Norwood event, a school reunion, were fully vaccinated. Former SA premier Jay Weatherill was among them and has tested positive.
The cluster has forced 210 close contacts to quarantine for seven days but officials expect that number to increase.
"It's quite clear we do have COVID in our state now," Chief Public Health Officer Nicola Spurrier told reporters on Thursday.
"This looks as though it was linked to someone coming from NSW. But I think there will be other chains of transmission."
Among the close contacts forced into isolation are Governor Frances Adamson, Opposition Leader Peter Malinauskas and his deputy Susan Close.
They have all tested negative, but under current rules must isolate for seven days.
Because of the virus scare, SA Health initially advised the parliament it should not sit but after investigations, the afternoon session on Thursday - the last before next year's election - went ahead.
A number of exposure sites linked to the new cases include the school reunion's theatre venue, restaurants and cafes, the Mt Barker Hospital and a caravan park.
Premier Steven Marshall said the spike in virus cases was not unexpected after the state dropped most of its border restrictions last month.
"I'm sure there are people anxious about opening up the borders. But I'm quite sure we are COVID ready," he said.
"We're on track at the moment. The cases were inevitable and now we're responding."
As part of that response and amid concerns over the Omicron variant of the virus, SA has moved to tighten its border rules with NSW, requiring all arrivals to have a COVID-19 test.
SA Health has also taken steps to be able to quickly identify the new variant should it arrive.
Police Commissioner Grant Stevens said officials were watching developments with Omicron, meeting several times a day.
He said it remained a concern to SA that some other jurisdictions only require international arrivals to quarantine for 72 hours or allow them to immediately travel to other places in Australia.
Under current rules, all international arrivals in SA must quarantine for 14 days.
But overall, Mr Stevens said SA's spike in virus cases was not unexpected.
"I went into this current phase of our COVID response not knowing what it would look like," he said.
"So I can't say I'm surprised we've got 20-odd cases now.
"When you take a step back and have a look at how things are going at the moment, I'd have to say we're on track and things are playing out as we would have hoped."
Australian Associated Press