Health officials do not believe the woman has been infectious since returning, but have identified a small number of people she has come into contact with.
“She did everything she was supposed to do when she returned to Australia,” ACT Health said in a statement on Monday.
Canberra has recorded three coronavirus deaths and 103 of the territory’s residents have fully recovered.
The ACT last Thursday had become the first jurisdiction to declare itself virus-free.
The overwhelming majority of Australian coronavirus cases have stemmed from returning overseas travellers, as has the ACT’s latest.
Acting Immigration Minister Alan Tudge said reopening Australia’s international borders was still a long way off.
Industry Minister Karen Andrews is hopeful a vaccine will be ready by the end of this year or early next year.
“Quite frankly, until such time as we have a vaccine, life is not going to return to normal,” she said on the Gold Coast on Monday.
Ms Andrews said Tuesday’s national cabinet meeting of federal and state leaders would focus on restrictions, but cautioned against a wide-ranging relaxation of rules.
“It’s very important that we all take baby steps,” she said.
Some restrictions on ACT residents were rolled back after the claim the territory was virus-free last week.
Canberra residents can now visit friends and family in NSW.
ACT Chief Minister Andrew Barr will also allow two adults and their children to visit other households within the territory while encouraging more retailers to reopen.
“It is okay to go shopping for items outside of what you would find in a supermarket,” Mr Barr said last Friday.
The territory wants public schools to return to face-to-face teaching, but not for the next little while.
“We are preparing to move to face-to-face delivery during term two if the circumstances allow us to do that sensibly,” Mr Barr said.
“This is likely to involve a staged return in public schools, beginning with younger children who need more hands-on support for learning and secondary students at key points in their learning journey, such as year seven and year 12.
“Teachers and school leaders will also need some time before students return to prepare and plan staggered lunch breaks, for example.
“Parents will have plenty of notice of these changes.”
Although the ACT is loosening some social and business restrictions, Mr Barr remains concerned about fresh waves of coronavirus infections.
“This virus will continue to circulate around the globe. There’s no immunity, there’s no vaccine,” Mr Barr said.
The change to allow children to accompany two adults visiting other households came into effect at midnight on Friday.
Mr Barr encouraged people travelling to NSW to do so in small groups: “So, (just) your household.”