There are now 17 cases linked to an outbreak in Adelaide's northern suburbs. Two schools and a fast food restaurant have been closed after cases were linked to the venues.

ACT Health has urged Canberrans to avoid non-essential travel to South Australia as authorities try to get case numbers under control.

In a statement, ACT Health said the situation was being monitored closely and the extent of the outbreak would be assessed.

The travel advice came as the ACT reported 24 days without a new infection and more than two-weeks without an active COVID-19 case.

More than 113,400 tests have been conducted throughout the pandemic, including 214 in the past 24 hours.

SA Health has released a list of bus routes and retail outlets visited by potentially infectious patients.

ACT Health said, "If you have been in any of these identified locations and recently returned, be aware of COVID-19 symptoms. If any symptoms develop, no matter how mild, immediately get tested and self-isolate until you receive a negative result. Further information on testing sites is on the COVID-19 website."

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It casts serious doubt over all other states and territories reopening their borders by Christmas as planned.

Tasmania, Queensland and Western Australia and the Northern Territory have reimposed border restrictions on SA residents due to the outbreak.

All arrivals to WA must now have a COVID-19 test and isolate for 14-days. Anyone entering the Northern Territory or Tasmania from SA must now also go into quarantine.

Adelaide residents entering Queensland from midnight will be required to quarantine as it declared the city a coronavirus hotspot.

Queensland chief health officer Jeanette Young said there had been a rapid rise in cases from four to 17 but it was unlikely the virus had been circulating prior to last Monday.

"Whether or not you've got symptoms anyone who has been in Adelaide since Monday of last week ... should go and get themselves tested," she said.

"I think it is unlikely virus was circulating prior to last Monday, I think that's a very cautious time frame."

Dr Young said her state was in a stronger position than early in the pandemic as border controls meant authorities had the details of anyone who had entered from SA.

"So we're able to contact those people and make sure they get themselves tested," she said.

But Premier Gladys Berejiklian has confirmed NSW will not close to South Australia and urged other states to have faith in their health systems.

The source of the outbreak has been traced back to one of the state's quarantine hotels. Fifteen of the cases are believed to be in the same family.

SA chief public health officer Nicola Spurrier said new cases had been picked up on Saturday after an 81-year-old woman went to Adelaide's Lyell McEwin Hospital for testing and was hospitalised,

Two of her family members, a woman in her 50s and a man in his 60s, also tested positive.

One of them worked in a hotel used by returning travellers and local residents who cannot quarantine at home.

Federal Health Minister Greg Hunt is confident South Australian authorities will bring the outbreak under control.

He has offered to send Australian Defence Force troops and a national incident centre is being set up.

"If more is required, more will be provided," Mr Hunt told the ABC on Monday.

"But these are the sorts of challenges that if we trade or engage with the world, if we bring Australians home, we will face, in a world where there's over half a million cases a day.

"Having these strong testing, tracing and isolation systems are absolutely critical and South Australia - on all the evidence - does have exactly that."

Prime Minister Scott Morrison said the Adelaide outbreak was a reminder to all Australians.

"Even after a lockdown, even after all of this time, the virus hasn't gone anywhere and it can be activated," he told 3AW radio.

"That's why none of us can be off our game, you've got to stay match fit on this all the time."

- with AAP