Victoria's streak of 61 days without a single case of Covid-19 in the community has come to an end.
Three cases — two women in their 40s and a woman in her 70s — were reported to authorities in Melbourne on Wednesday afternoon, none of which are in hotel quarantine.
Authorities have already been in touch with all three cases' close contacts and advised them to isolate.
The three cases are all in Melbourne's southeast.
A spokesperson for the Victorian Government says the three new cases are believed to be linked to the Sydney cluster.
Victoria's Commander of Covid-19 response, Jeroen Weimar, said in a statement that close contacts were identified.
"We have three cases that are self-isolating at home and are being supported and monitored … We have been in this position before and we are deploying our full outbreak approach around these cases.
"Extensive contact tracing is underway and as a result there are currently more than 40 primary close contacts that are being supported to isolate."
New York Times tennis writer Ben Rothenberg tweeted that the news was a "blow to AusOpen planning to have Melbourne's fragile zero cracked".
It comes as the state tightens its border with NSW where a cluster on the northern beaches is now just one of a number of hot spots.
NSW Premier Gladys Berejiklian earlier today described an outbreak in Croydon in Sydney's inner-west as "concerning", with six cases recorded so far. There has also been a further infection in Wollongong, 120km south of Sydney, taking the total number of cases there to two.
But things quickly escalated after her press conference. A pharmacy in the Blue Mountains suburb of Katoomba had to break the news itself on social media after a staff member became infected with Covid-19.
On Facebook, staff at the Greenwell and Thomas Pharmacy at Katoomba, 110km west of Sydney, wrote that the infected staff member worked on Saturday, December 26 and Sunday, December 27.
People infected with Covid-19 visited a host of venues in southwest Sydney, too, according to the latest alert from NSW Health on Wednesday afternoon.
The suburbs of most concern now include Bankstown, Bass Hill, Punchbowl and Liverpool.
Berejiklian pleaded for anyone in the state with even the mildest of symptoms to come forward for testing in a bid to get on top of the outbreak.
She also announced an urgent tightening of restrictions for New Year's Eve across Greater Sydney with indoor gatherings limited to just five people and outdoor groups restricted to 30 until further notice.
"Our preferred advice is that people just stay home for New Year's Eve, but if (you) must have people over, don't have more than five and please make sure you have adequate social distancing, good ventilation, all those things Dr Chant and the health experts talk about," she said.