Australia's capital city, Canberra, announced a snap one-week lockdown from Thursday evening after reporting its first locally acquired case of Covid-19 in more than a year.
Authorities later confirmed an additional three cases, all close contacts of the first case, an unnamed man.
The move came as the New South Wales state government said extra military personnel may be called in to ensure compliance with lockdown rules in Sydney, around 160 miles to the northwest of Canberra.
"We are making sure that we do not leave any stone unturned in relation to extra (military) resources," New South Wales (NSW) state Premier Gladys Berejiklian told a media conference in Sydney, the state capital.
Some 580 unarmed army personnel are already helping police enforce home-quarantine orders on affected households in the worst-affected suburbs of Sydney, Australia's most populous city.
Several regional towns scattered across NSW have also been forced into snap lockdowns after fresh cases, raising fears the virus is spreading out of control.
Despite seven weeks of lockdown in Sydney, daily infections continue to hover near record highs. NSW on Thursday reported 345 new locally acquired cases, most of them in Sydney, up from 344 a day earlier.
Lockdown rules were tightened in three more local council areas in Sydney, limiting the movement of people to within 3 miles of their homes.
Officials also reported the deaths of two men in their 90s, taking the total deaths in the latest outbreak to 36. A total of 374 cases are in hospitals, with 62 in intensive care, 29 of whom require ventilation.
In Canberra, authorities said the one-week lockdown was needed as they were unsure how the man is his 20s acquired Covid-19.
Download the NBC News app for full coverage of the coronavirus outbreak
Canberra has largely escaped any cases since the beginning of the pandemic, and confirmation of a Delta variant saw panic buying at the supermarkets and long lines at testing sites.
Neighbouring Victoria state on Thursday reported 21 new locally acquired cases, up from 20 a day earlier, as 5 million residents of Melbourne, the state capital, prepare to enter a second week of lockdown.
Of the new cases, six spent time outdoors while infectious, a number which authorities have said must return to near zero before restrictions can be eased.
Australia has largely avoided the high coronavirus numbers seen in many other countries, with just over 37,700 cases and 946 deaths, and several states remain almost Covid-free despite the outbreaks in Sydney and Melbourne.
But the rapid spread of the Delta variant in New South Wales and a slow vaccine rollout has left the country vulnerable to a new wave of infections.
Only around 24 percent of people above 16 years of age are fully vaccinated.