ACT chief health officer Kerryn Coleman said authorities believe the woman in her 20s was exposed to the virus overseas.

She travelled on flight QF94 from Los Angeles to Melbourne, which arrived at 8.30 am on Tuesday, and then flew from Melbourne to Canberra on flight QF2130 arriving at 1.30pm.

Anyone seated in rows 71-75 on the international flight and 15-19 on the domestic flight are considered close contacts and have been asked to self quarantine and contact health authorities.

"She has complied with self-quarantining directions for travellers returning from overseas and is now isolating with ACT Health support," the statement said.

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ACT Health will be working with national authorities to identify individuals who are close contacts as a priority, a statement said.

Anyone else travelling on these flights in other rows are not considered to be a close contact, and as a precaution, should monitor their health until 31 March. Self-quarantine is not required.

A small number of household close contacts have been identified and contacted on Thursday afternoon afternoon.

It comes after a woman in her 70s and two men in their 30s were diagnosed with the virus in the ACT.

There are currently four confirmed cases of COVID-19 in the territory, with 1,853 negative tests so far.

Any Canberrans sitting in the rows mentioned are asked to quarantine themselves and call the Communicable Disease Control Section of ACT Health on (02) 5124 9213 for further advice.

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