Australia's New South Wales (NSW) reverted on Friday to making the wearing of masks mandatory on public transport in Sydney, as a cluster of the highly-infectious coronavirus Delta variant expanded to a fourth person.
Authorities said all planned outdoor events with good Covid-19 safety plans can proceed in the country's largest city.
The latest cluster, the first in the state in more than a month, was traced back to a driver who occasionally used to transport overseas airline crew.
Health officials said the latest case likely picked up the virus through minimal contact with an infected person in a Sydney shopping centre.
"We don't want people panicking but at the same time we want everyone on high alert," NSW state Premier Gladys Berejiklian told reporters in Sydney.
Masks will be made mandatory in trains, buses and ferries from 4 pm local time (0600 GMT) on Friday for five days while officials urged Sydney's five million residents to wear them in all indoor venues including supermarkets and theatres.
"We feel the response we have outlined today is enough for the information we have today ... we have room to move if the situation worsens or something unexpected happens," Berejiklian said.
Australia has successfully contained all past outbreaks through tough social distancing rules, snap lockdowns and internal border controls helping keep its Covid-19 numbers relatively low, with just over 30,300 cases and 910 deaths.
Though no local cases were reported in the country for most days this year, the nationwide vaccination drive has hit another roadblock.
In the latest setback, Australia on Thursday amended its immunisation policy by limiting AstraZeneca's Covid-19 shots only to people above 60 years over concerns of blood clots, while the Pfizer vaccine is recommended for other eligible age groups.
This could hold back an already sluggish rollout as Australia has to rely on other countries for its Pfizer shots while the AstraZeneca doses are manufactured locally.
Only 4% of Australia's adult population of 20 million have been fully vaccinated while about 25% have had at least their first dose.