Stay-at-home requirements have been imposed on Canberrans returning from parts of Sydney as authorities tackle a "very concerning" coronavirus outbreak.
Days before the school holidays kick off in the ACT and NSW, Canberrans have been told to stay away, as a further 23 cases of coronavirus were recorded in the past 24 hours in Sydney.
"This is a situation that has the potential to become extremely serious," ACT Health Minister Rachel Stephen-Smith said.
"This kind of thing seems to always happen as we lead into school holidays, or a public holiday or long weekends, and yet again, we are going to see Canberrans and people from Sydney having their travel plans interrupted."
Anyone who enters the ACT from any of the seven listed council areas from 4pm Wednesday will be subject to stay-at-home requirements.
These council areas are: City of Sydney, Waverley, Randwick, Canada Bay, Inner West, Bayside and Woollahra.
Hundreds of thousands of people in those jurisdictions will be confined to metropolitan Sydney, as NSW Premier Gladys Berejiklian banned non-essential travel for those residents.
People in greater Sydney, Blue Mountains, Central Coast and Shellharbour regions are banned from having more than five visitors to a household.
The NSW Premier said lockdown wasn't off the table.
"We know basically where the super-spreading events have been, we know where the virus is circulating, and we don't want to take any further action than what we have now," Ms Berejiklian said.
Ms Stephen-Smith said anyone in the ACT who has been in the greater Sydney, Central Coast, Nepean Blue Mountains, Wollongong and Shellharbour regions of NSW, in the past 14 days, must complete an online declaration form.
They should also get tested for COVID-19 if they have the "mildest symptoms".
She said non-ACT residents who had been in the identified seven council areas from 4pm Wednesday, couldn't enter without an exemption.
The requirements will remain in place for at least a week until 11.59pm June 30.
ACT Chief Minister Andrew Barr said the announcement mirrored NSW requirements.
He said Canberrans should not travel to Sydney at this time.
"If you must travel for essential reasons, it's important to be mindful of public health directions in place in NSW and here in the ACT," he said.
"This includes the almost-certainty of having to change your travel plans and adhere to many public health instructions on your return.
"The best and simply the easiest way you can help now is to not travel to Sydney."
Anyone who is subject to stay-at-home requirements from 4pm Wednesday must complete an online declaration form within 24 hours prior to arriving in the ACT, or within 24 hours from the time it is available.
When they arrive, they must travel directly to their residence and can only leave for approved essential purposes.
Essential purposes include shopping for groceries or necessary supplies, essential work that can't be done from home, to get medical care or be vaccinated for COVID-19.
Anyone under those requirements aged 12 and over must wear a mask when leaving their home for any of those essential reasons.
A mask isn't required for people doing vigorous exercise outdoors.
Police checks at the ACT border will not be in place but haven't been ruled out should the situation "escalate dramatically".
"If we are getting information that indicates we need to see more enforcement ... we have done that in the past, and we have that option available to us, but it is resource intensive, " Ms Stephen-Smith said.
The Sydney outbreak has escalated dramatically since an unvaccinated Sydney airport limousine driver last week tested positive to the virus, which quickly spread at Bondi Junction's busy Westfield shopping centre.
There are now 31 cases linked to the Bondi cluster.
The seven council areas under restrictions have also been deemed Commonwealth virus hotspots.
NSW recorded 10 locally acquired cases to 8pm on Tuesday, seven of which were already announced. An additional 13 cases were reported after 8pm, which will be counted in Thursday's official tally.
Four of these cases are not currently linked to existing cases.
Ms Stephen-Smith said NSW had a strong track record of contact tracing, but the growing cluster was of "significant concern".
"That fact that this is the delta variant, that we're seeing transmission in the ways that we have in the Sydney cluster is very concerning."
Eight of the 13 cases confirmed overnight were at a birthday party attended by about 30 people on Saturday, which NSW Chief Health Officer Kerry Chant described as a "super spreader event".
An asymptomatic worker from Bondi Junction went to the party, as did a father and a daughter. The infected child then went to her childcare centre 60km southwest of Sydney's CBD on Monday, which has been closed for deep cleaning.
Dr Chant warned there were people "unknowingly" infected in the community and "a very real and present danger" existed across Sydney.
- With AAP
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