Just days after Prime Minister Scott Morrison announced plans to reopen Australia's internal borders by Christmas, an exploding coronavirus cluster has plunged those plans in limbo.

Going from bad to worse, South Australia's cluster ballooned from three cases of Covid-19 yesterday to 17 cases by Monday morning.

The outbreak triggered warnings and closures across Adelaide, and is now sparking panic interstate.

Western Australia Premier Mark McGowan on Monday afternoon reintroduced a hard border closure for the neighbouring state, starting from 6pm except for "strict exemption" categories.

Queensland has announced they too will be closing off the border to parts of South Australia from midnight tonight.

South Australia Premier Steven Marshall on Monday afternoon said the state is facing its "biggest test to date" and the border arrangements for other jurisdictions are their decisions to make.

"We can and we must rise to this new challenge," he said.

"I want to assure all South Australians that we are working around the clock to stay ahead of this cluster. No effort will be spared to slow and stop the spread of the Parafield cluster.

"I said from day one that we will not hesitate to act on the expert help advice because keeping the people of South Australia safe and strong is our unequivocal priority.

"Time is now of the essence and we must act swiftly and decisively. We can't wait to see how bad this gets."

Marshall said the state has been the "recipient" of data from other health administrations around the country and is now "transmitting it".

He announced all inbound international flights to Adelaide have been suspended, at the state government's request, for the rest of the week.

"Our priority is ensuring we have enough capacity in medi-hotels to meet quarantine requirements," Marshall said.

The Premier introduced a raft of new restrictions for the next two weeks and also advised "against any unnecessary travel at the moment".

"We're not giving a direction that you can't go on any travel," he said, noting that if it is "spontaneous and can be avoid", the Government is saying to people "don't proceed with it".

Marshall said as of 2pm Australian Central Daylight Time, there remained 17 cases in the cluster and all are "stable".

South Australia's chief public health officer Professor Nicola Spurrier said after initial review, the origin of the cluster is "clearly from a medi-hotel" and they are awaiting genomic testing results from two security guards at the medi-hotel and one person working in "back of house".

Meanwhile, Morrison said he spoke to Marshall "first thing" on Monday morning and the state's contact tracing and testing will "now be put to the test"

He said there had been a " spectrum of responses" from other states and territories but he understood those in Tasmania and the Northern Territory would be "temporary responses".

"What's important is these don't get sort of locked in as part of another enduring disruption and as soon as South Australia is able to get on top of this I would be expecting states would keep on the path that we have set towards Christmas," Morrison told reporters in Melbourne.

Asked whether the outbreak "jeopardises" his chance of having borders opened by Christmas,Morrison replied: "Well, I hope not."

"We have all said that these are subject to the health conditions and what occurs, but I would hope not," he said.

"So far, it is an early good response, but we are certainly not going to get ahead of ourselves as to how we anticipate this will play out in the days ahead."

Speaking to media on Monday, Queensland's chief health officer Dr Jeannette Young said the revised border closure would impact residents from Adelaide only, and not those coming in to the Sunshine state from regional South Australia.

"I have been recommended that all of Adelaide … and the local government areas be made a hotspot, and as of 11.59pm tonight, anyone who comes into Queensland who has been in that part of South Australia since Monday of last week will need to go into hotel quarantine for 14 days," she said.

"I am also asking that anyone who has arrived in Queensland who has been in Adelaide since Monday of last week to immediately come forward and get themselves tested and go into quarantine wherever they are."

Queensland still has border restrictions in place with Greater Sydney, despite the state not recording any local cases of Covid-19 for more than a week.

Annastacia Palaszczuk, Premier of Queensland, also said that those who arrived into the state from last Monday will need to self-isolate while they await a test result.

The announcement comes as Northern Territory's Chief Minister Michael Gunner closed the borders to the Top End to the entire state of South Australia "effective immediately".

Addressing media on Monday morning, Gunner said the decision to close the border to all of South Australia was to manage a "critical point" in the state's contact tracing process.

"It is what we don't know that worries us the most," he said.

"We are declaring South Australia a hotspot for travel to the Northern Territory effective immediately. That means that people who arrive here from South Australia this morning will be directed to supervised quarantine or given the option of returning to South Australia. "People who intend to travel here later today in South Australia will need to make a decision now – to stay there, or if they come here, to enter supervised quarantine."

Gunner said that because of the late notice behind the decision, those who enter the Northern Territory today or tomorrow from South Australia will not need to pay the $2500.

Moments later, Tasmania's Premier Peter Gutwein announced his state would also be implementing new border measures with visitors from South Australia, urging anyone who had entered since November 8 to self-isolate at their hotel or place of residence for 14 days.

Gutwein said any further decisions on his state's border with South Australia would be announced later today.

Victorian Premier Daniel Andrews, whose own state has come out the other side of a devastating second wave of the disease, has followed suit.

Andrews this morning declared South Australia a hot spot, with any arrivals to Melbourne Airport to undergo interviews upon their arrival, and also may be required to get tested.

Victoria's border, though, will remain open for now.

In the ACT, the Territory's health department said in a statement this morning that they're "monitoring the Covid-19 situation in South Australia closely".

"For the time being, we are asking ACT residents to reconsider non-essential travel to South Australia while SA Health continues its investigations and, here in the ACT, we assess the extent of this outbreak," ACT Health said.

NSW Premier Gladys Berejiklian, however, said her state would not be making any changes to current border arrangements, meaning residents flying in from South Australia can enter the state freely without going in to mandatory quarantine.

"We need to learn to live with Covid," Berejiklian said.

"You can't shut down borders and disrupt lives every time there is an outbreak and disrupt businesses. We need to have confidence, not just in our own system, but the system in other states to be able to get on top of the virus.

Berejiklian said she had been in contact with her state's chief medical officer Dr Kerry Chant, who will be meeting with state and territory counterparts at midday.

"We would be arguing that the [outbreak] is no reason to shut off New South Wales citizens from the rest of the country," she said.

"I don't think it is a sensible approach moving forward to shut your borders every time there is an outbreak.

"Obviously if the numbers were in hundreds or there were concerns … of course we would look at our situation, but New South Wales will not be moving as other states have.

"At the end of the day, we need to live with the pandemic and that is why I say to every state, have confidence in your system. We can all support each other and work together rather than opening and shutting borders, which is no way to live, frankly."

Passengers flying into Perth from Adelaide on Sunday evening received a shock on landing after being told to either adhere to strict new coronavirus measures or fly back home.

WA Premier Mark McGowan updated the state's border measures on Monday afternoon, closing the border to travellers who have been in South Australia for the previous 14 days after the chief health officer categorised the state from "very low risk" to "medium risk".

The previous measures, only introduced on Sunday, had required anyone arriving in Perth from South Australia to be tested for Covid-19 and self-quarantine for 14 days.

They were so abrupt, one Qantas flight was mid-air when the measures were introduced and passengers were given the news from health officials at the airport upon landing.

"The community outbreak of Covid-19 in South Australia is extremely serious," McGowan said on Twitter on Monday.

"Yesterday, we acted swiftly to strengthen our controlled border regime.

"Today, on further advice from our CHO (chief health officer), we have decided to take further steps to protect our state."

McGowan said as of 6pm Monday, people will need to meet a "strict exemption category" if they want to enter WA from South Australia.

"These categories include compassionate reasons – and that could include those Western Australians that may have travelled to South Australia in the last few days," he said.

Federal Health Minister Greg Hunt has confirmed the Government has offered help to South Australia after the state's cluster grew to 17 cases.

"This is obviously a matter of concern. We've offered the Australian Defence Force and any resources that are required will be provided," Hunt told Sky News.

"We're confident that South Australia has outstanding testing, contact tracing and isolation capability.

"But these are exactly the types of incidents for which Australia has been preparing for."

Speaking to Adelaide's 5AA Radio on Monday morning, Spurrier confirmed the cluster was growing at a rapid rate, describing the outbreak as "very serious".

Meanwhile, given Victoria has recorded their 17th day of no coronavirus cases or deaths, Gunner said NT Melburnians would be able to enter the Top End without needing to quarantine later this month.

"Two weeks ago today, we started to welcome regional Victorians back to the Territory without needing to quarantine. We still said that we are monitoring Greater Melbourne and weren't ready to lock in a date to remove the hotspot," Gunner said.

"But in the 14 nights since then, there have been zero new cases in Melbourne or anywhere else in Victoria.

"If you are as safe as we are, then you are welcome here. Melbourne is safe."

Melburnians will be welcomed back to the Territory from November 30.

Last Friday, it was announced South Australia's border with Victoria will reopen on December 1.

Asked on Monday if that will still go ahead, Spurrier said her focus since Saturday night "has been getting in front of this cluster".

"I'm aware, obviously, that border measures have been put in place by other states," she said.

"Look, you know, that date is a little far off but the risk for us is within our own community. This is where we need to be focusing at the moment."