MELBOURNE (BLOOMBERG) - The Australian authorities are racing to determine the source of a small cluster of coronavirus cases in Melbourne, the first community infections in the nation's second-most populous city in almost three months.

The four cases are close family members spread across two households in the city's northern suburbs, Victoria state Heath Minister Martin Foley told reporters on Monday (May 24). The city of about five million people has largely contained the virus after enduring one of the world's longest and most stringent lockdowns last year.

"After nearly three months of no community transmission and a degree of complacency having settled in, this is a reminder" that the pandemic is not over, Mr Foley said. "We are a long way from herd immunity in Victoria and Australia."

Australia has been successful in controlling the virus through rigorous testing and contact tracing and by closing its international border to non-residents - other than a new travel bubble with New Zealand. But cases have sometimes leaked into the community from hotels where returned overseas travellers have to quarantine for 14 days.

Prime Minister Scott Morrison's government is under pressure to speed up the pace of the vaccine roll-out. About 3.6 million people in the nation of 26 million have received their first jab.