Picture used for illustrative purposes only.

Sydney reported a record daily number of new coronavirus infections on Thursday, as Melbourne announced a sixth lockdown for the city. Australia struggles to contain an outbreak of the highly infectious Delta variant.

Victoria premier Dan Andrews said he had "no choice" but to make the "very difficult announcement" to lock down Melbourne and the rest of the virus-weary state little more than a week after the last lockdown ended.

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"None of us are happy to be here, none of us," he said, citing the danger posed by eight "mystery" cases that have yet to be traced.

"There is no alternative to lockdown" he added.

Sydney is nearing the seventh week of its own nine-week lockdown.

"The alternative is we let this run that gets away from us, and our hospitals will be absolutely overwhelmed. Not hundreds of patients but thousands."

Sydney, Australia's largest city and the capital of New South Wales state, is nearing the seventh week of its own nine-week lockdown. It reported five deaths and a record 262 infections in the past 24 hours.

Victoria state said it has detected eight new cases, the bulk of which could not be traced back to a known infection, triggering a one-week lockdown for the state's more than 6 million people from later on Thursday evening.

"Nothing about this is optional. This is a lockdown, it will be enforced, for the best of reasons and the best purposes, to bring these case numbers down, under control so we can once again be open," Victoria state Premier Daniel Andrews told reporters in Melbourne.