Canberrans will only be able to leave home for essential reasons from 5pm today (local time), Chief Minister Andrew Barr has confirmed.
The lockdown follows a case being out and about while infectious. The source of the infection remains unknown.
Covid-19 has also been detected in wastewater.
Masks will be mandatory, general retail will shut and hospitality venues will only be able to operate takeaway services.
"We have said throughout the outbreak in Greater Sydney that we would act quickly and decisively," Mr Barr said in a statement.
"We have seen that a short and immediate lockdown limits the potential spread of the virus, and is the best path to avoiding longer and more damaging lockdowns."
People can still leave home for essential work, healthcare including to receive a Covid-19 jab, an hour of exercise a day and to buy essential supplies.
Both houses of federal parliament were already set to rise on Thursday evening.
There is another fortnight of sittings scheduled for August 23 to September 2.
Many politicians are appearing remotely because of lockdowns and border closures down the east coast and Parliament House is shut to the public.
Canberra's lockdown follows the spread of cases into western and northwest NSW.
Stay-at-home orders were on Wednesday announced for the Dubbo, Bogan, Bourke, Brewarrina, Coonamble, Gilgandra, Narromine, Walgett and Warren local government areas.
The ACT had remained largely untouched by the virus since its last locally acquired case in July last year.
