ACT health authorities initially revealed a man had spent four days out and about while infectious.

His infection is thought to be from Sydney's outbreak, sparking the ACT's first case in 13 months and a seven-day lockdown from 5pm on Thursday.

Authorities updated their advice just as the lockdown began to announce three more cases, all close contacts of the man.

Treasurer Josh Frydenberg acknowledged the ACT's "tough news" in parliament and confirmed federal support would extend to Canberra businesses and individuals.

Parliament House will remain open for MPs and senators who stay in Canberra ahead of the next sitting fortnight scheduled for August 23.

Both houses are rising to coincide with the imposition of local restrictions and the federal government is helping arrange flights for parliamentarians who want to go home.

NSW has told ACT residents to stay away and will impose stay-at-home orders on anyone who was in Canberra since August 5.

The Northern Territory has declared the city a hotspot. Victoria, South Australia and Western Australia's borders were already shut to Canberra.

ACT health authorities are looking into how the man, from the northern suburb of Gungahlin and aged in his 20s, became infected.

It's thought he has the Delta strain currently traversing Australia's eastern seaboard and stretching into western NSW.

He was out and about between Sunday and Wednesday, including at a nightclub, then a church, the Canberra Outlet Centre as well as various other shops and venues.

The man is not believed to be vaccinated because of his age.

Vulnerable ACT residents, such as those with underlying health conditions, and those aged 30 or older are eligible for Pfizer.

The list of exposure sites is expected to grow and people caught up must get tested and quarantine for 14 days.

More than 1000 people are already quarantining after returning from NSW and authorities expect that number to rise significantly.

Chief Minister Andrew Barr told reporters Sydney's spiralling outbreak showed the need to move rapidly with significant restrictions.

"If the thought was that you could somehow have a perfectly sealed Canberra bubble, that was never possible," he said.

Masks will again be mandatory in the ACT, general retail will shut and hospitality venues can only do takeaway.

Schools will move to online learning except for the children of essential workers.

People not required to quarantine can only leave home for essential work, to buy supplies, for one hour of exercise and healthcare including getting a COVID-19 vaccination.

Full vaccination rates in the ACT are sitting at 27.5 per cent and first dose rates at more than 50 per cent.

"That's still nowhere near where we need to be to have much greater confidence around being able to manage outbreaks," Mr Barr said.

Prime Minister Scott Morrison told parliament one-quarter of eligible Australians will have been fully vaccinated this week.

Australian Associated Press