Case numbers quadruple in just one day.

The South Australian outbreak has significantly increase to 17 new cases overnight.

It began with a woman in her 80s, who caught the virus from an international traveller at the hotel she worked at.

SA has not recorded community transmission since April.

The woman has since passed the virus onto 15 family members.

SA chief health officer Nicola Spurrier revealed today that Port Adelaide Hungry Jacks, Mawson Lakes Primary School, Preschool and the Parafield Plaza Supermarket have been closed in connection with the outbreak.

Ms Spurrier said the situation was “very serious”.

In response, WA has re-enforced quarantine restrictions on arrivals from Adelaide.

Anyone who arrived WA from Saturday 12am was forced to immediately self-isolate for two weeks, including about 500 people who landed in Perth airport over the weekend and further 439 who entered the State by road.

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The restriction will not be lifted until SA records no community transmission over 28 days.

Read more: Travellers From South Australia Will Now Have To Self-Quarantine For 14 Days

Victoria today recorded its 17th day free of new COVID-19 cases, with NSW also going more than a week without new infections.

But NSW Premier Gladys Berejiklian warned she expected more cases over coming weeks.

Health Minister Greg Hunt said there was no medical advice supporting the swift move.

“We don’t have any advice from the Chief Medical Officer that there’s a base for any state and territory not to be open to any state and territory at this time,” he said on ABC.

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“I’ve had a briefing this morning from the National Incident Centre and last night from the Chief Medical Officer of Australia, Professor Paul Kelly, and their advice is that the system in South Australia is strong.”

He said the change in restrictions would be “immensely disappointing” for travellers.

It comes as Prime Minister Scott Morrison on Friday urged WA to “remain safely open” from here on out.

More Perth news on SoPerth.com.au.