South Australia recorded 17 locally acquired coronavirus cases on Monday, one day after the state recorded its first community transmitted infections in months.
It's prompted the Northern Territory (NT) and Tasmania to close its borders to the state to help stop the spread of the virus.
Anyone arriving into NT or Tasmania from South Australia is required to self-isolate upon arrival.
It comes after Western Australia Premier Mark McGowan imposed a 14-day quarantine on travellers arriving from South Australia on Sunday.
South Australia reported three community COVID-19 cases on Sunday after a worker from a quarantine hotel infected family members, health officials said.
That came just after Australia saw one week of no coronavirus community transmissions and no related deaths on Saturday for the first time since the start of the pandemic. Victoria, the country's COVID-19 hotspot, recorded its 16th consecutive day of no new cases and no deaths on Sunday.
Victoria deputy chief health officer Allen Cheng told a news conference on Saturday that the run of zero cases was "about as good as it can get".
Neighbouring NSW, the most populous state, saw eight days without local transmissions, while Queensland has had no new community infections for two months. Both states saw a handful of new cases in managed quarantine among travellers from overseas.
Australia has recorded about 27,700 infections of the new coronavirus and 907 COVID-19 deaths, a fraction of what many other developed nations have seen, thanks to an impressive early response and strict measures that included sending Victoria's Melbourne, the country's second-largest city, into months-long lockdown.
Victorians still must wear masks and public gatherings are limited, but more easing is expected in a week. Western Australia, which had its borders closed to the rest of the country for seven months, had them reopened on Saturday.
Reuters / Newshub.