Canberrans should avoid travel to Sydney ahead of the school holidays after the Bondi cluster grew to 21 cases.
Mandatory mask-wearing has been extended by a week for Sydneysiders on Tuesday as the state reported 10 locally acquired cases.
ACT Health Minister Rachel Stephen-Smith said Canberrans should avoid travelling to the greater Sydney region.
"We know how quickly these clusters can sometimes develop," she said.
"That can have impacts on people when they come back to the ACT of either having to be subject to stay at home or quarantine requirements.
"We already have a number of people in the ACT who are in quarantine as a result of these latest outbreaks. We'd like people to avoid travelling to Sydney [unless] they absolutely need to.
"We know how quickly these clusters can sometimes develop. That can then have impacts on people when they come back to the ACT of either having to be subject to stay-at-home or even quarantine requirements."
Ms Stephen-Smith said anyone who has recently returned from NSW should regularly check the ACT COVID-19 website and complete a declaration form if they have attended an exposure location.
"So that ACT Health has your contact details, has the details of where you've been, so that if there are any developments, they can contact you directly and tell you what you need to do as quickly as possible."
NSW Premier Gladys Berejiklian said masks will remain compulsory until midnight on June 30 on public transport and in most indoor settings for people in Greater Sydney, the Blue Mountains, the Illawarra and Shellharbour.
"It is only when you are eating or drinking indoors at a venue that you can't or shouldn't wear a mask," she said on Tuesday.
"In every other circumstance, if you live or are in Sydney, you must wear a mask for another week beyond Wednesday midnight."
"Unfortunately, a number of people who have contracted the virus have done so in a very fleeting way," she said.
She also flagged she was in the process of mandating the use of QR codes at all retail outlets.
When asked if another lockdown was looming she did not rule it out.
"If we suddenly have a number of unlinked cases and if we suddenly have them outside the geographic region they are concentrated in, that will obviously adjust the health advice and we will respond to that," she said.
NSW Health has also ramped up its advice around the original infection site at Westfield Bondi Junction and is asking anyone who was there - including the car park - at any time between June 12 and 18 June to get tested for COVID-19.
Only people with symptoms and those who have been to specific exposure venues at the listed times need to test and isolate.
Five new cases were detected in testing in the 24 hours to 8pm on Monday - two of those had already been announced and both people tested positive while in isolation.
An additional seven cases were diagnosed after 8pm on Monday and will be included in the official tally for the next 24-hour period.
The three new cases reported in the period to 8pm on Monday, included a woman in her 60s in the Illawarra and a woman in her 40s from Sydney's northern suburbs who had both been in isolation as they were close contacts of previously reported cases.
A woman in her 20s from Sydney's eastern suburbs who works at Bondi Junction also tested positive.
Testing numbers were up to 28,645 by 8pm on Monday night compared to 25,252 the previous day.
The premier said she expected household contacts who were already in isolation to test positive.
All of the new cases except for two were already in isolation when tested and all of the cases except for one, a student at St Charles Catholic Primary School in Waverley, were linked to existing cases.
Chief Health Officer Kerry Chant said the child had tested positive "a few hours ago" and the school was working with health officials to develop a plan for the 365 children in the eastern suburbs school to be "discharged home to their parents" and tested.
Dr Chant said this case and the woman who works at Bondi Junction were the only two cases officials were concerned about when it came to transmission.
NSW Health Minister Brad Hazzard said the Delta variant of COVID-19 was a "gold medallist" in jumping from one person to another and he encouraged people to continue using QR codes.
- with AAP
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