Preparation for a border bubble surrounding the ACT and neighbouring NSW towns has begun, should the risk of outbreaks across the border worsen.
NSW Deputy Premier John Barilaro said a border bubble, similar to what has been implemented on the NSW-Queensland border, was being planned.
"We've already begun preparation and work towards what a bubble would look like if and when we would need to see a strengthening or a lockdown of that border," he told ABC Radio Canberra.
"We are talking an extreme decision, [we are] not yet at that point but it is something we have to plan for."
Mr Barilaro said the next few weeks would be critical for NSW to flatten the curve.
"In Sydney, those hotspots are now becoming an issue, they are breaking out which means more cases overnight," he said.
"Another 20 yesterday shows that if you don't contain this 20 can become 50, can become 100."
More than 60 Canberrans are currently in isolation as a coronavirus cluster linked to the Batemans Bay Soldiers Club grew to eight cases on Monday.
The ACT chief health officer, Kerryn Coleman, labelled it the "biggest risk" to Canberra as it is the closest COVID-19 has come to the territory.
It has prompted the ACT government to urge Canberrans to reconsider the need to travel to the South Coast.
A public health direction came into force on Monday, legally requiring anyone who visited the Soldiers Club on Monday July 13 or Wednesday July 15 to Friday July 17, to self-isolate for 14 days.
Health Minister Rachel Stephen-Smith said a hard border closure would a be last resort to defend the territory from a rising NSW caseload, but if the situation worsened significantly other border control measures could be imposed to incorporate neighbouring towns.
"We are really conscious that the workforce flows both ways from Queanbeyan, Jerrabomberra, Googong, Murrumbateman, even Bungendore," she said.
Ms Stephen-Smith said discussions had begun with local councils and NSW Cross Border Commissioner James McTavish about restricting travel to the broader Canberra region if growing community transmission from NSW outbreaks threatened.
She was confident NSW health authorities were acting quickly to trace lines of transmission and would not point to specific triggers that could set border controls in motion.
However, it would be a staged approach, Ms Stephen-Smith said.
"We would consider then putting a personnel presence on the border but not necessarily closing the border."
Yass Valley Council Mayor Rowena Abbey welcomed the idea of a "safety bubble" which would allow life as normal to continue for around 3500 Yass Valley residents who commute to the ACT daily.
"We are a bit of a bubble around here from a planning perspective and a health management perspective," she said.
NSW reported 20 new cases in the 24 hours to 8pm on Sunday, all linked to known clusters - the Crossroads Hotel (now 48 cases), the Thai Rock restaurant and the Batemans Bay Soldiers Club.
NSW Premier Gladys Berejiklian said the next few weeks were critical. She urged people to avoid large crowds and think twice about going into any crowded place even with social distancing.
"I cant say enough how stressed and worried I am about the next few weeks," she said.
"We are at a critical point in our state's ability to deal with the pandemic ...
"All of us have to really think about what we're doing in the next few weeks."
If NSW got through the next three for four weeks and stayed on top of the seeding from Victoria and current outbreaks, the state could have "medium to long term surety" that it could keep moving forward, she said.
It came as the ACT recorded a tenth day without a new coronavirus case and the active caseload dropped to three. The first five cases reported in the ACT in more than one month were all linked to the Melbourne outbreak.