Canberrans have been told not to travel to coronavirus hotspots in Sydney.
It comes as the ACT records its fourth day without reporting a new coronavirus case.
Health Minister Rachel Stephen-Smith said no one should be travelling to the Sydney locations of Liverpool or Campbelltown.
"We know that the outbreaks in Victoria and NSW are of real concern," she said.
"I want to assure everyone that we are in, and remain in, a good position to respond to any new case of COVID-19 here in the ACT."
Ms Stephen-Smith warned any travel to other jurisdictions could be interrupted at any time.
"You do need to be aware the circumstances at this point can change very quickly and you could be caught up in some changes in direction," she said.
"That particularly applies to people who might be travelling to Sydney or NSW."
Ms Stephen-Smith said while it would be an "absolute last resort", she could not rule out a border closure with NSW if the situation in the state deteriorated.
Chief health officer Kerryn Coleman said travel bans may be put in place if there was an uptick in community transmission in Sydney
"When we start to see a number of cases that are getting notified where the sourced for that case cannot be identified then we do become concerned there are cases within the community and we don't know where they are," she said.
Dr Coleman said there would be more discussion about the merits of pursuing elimination, instead of suppression, at national cabinet.
The ACT announcement came after Queensland said it would ban people coming into the state from the two hotspots.
There are five active cases of COVID-19 in the territory, with the total tally at 113.
All cases currently in the ACT are linked to the Melbourne outbreak after two people returned from a hotspot, and a further three cases were identified as close contacts.
It comes as NSW reported 13 new cases on Tuesday.
There are now 30 cases linked to an outbreak from Casula's Crossroads Hotel. It has prompted the government to implement stricter restrictions on pubs.
Canberrans who visited the Crossroads Hotel between July 3 and 10, and anyone who visited the Picton Hotel on Saturday July 4, Thursday July 9, or Friday July 10, are urged to isolate immediately and get in touch with ACT Health through the COVID-19 Helpline on (02) 6207 7244.
These people are also encouraged to get tested for COVID-19, regardless of whether they are showing symptoms.
ACT Health said people should only present for testing if they had symptoms or had visited those venues during the times listed.
Canberra residents travelling in NSW are also reminded to be aware of the situation in that state and take appropriate precautions.
This includes maintaining appropriate physical distancing measures and practicing good hand hygiene.