On Sunday, Arcare Maidstone was locked down and residents were placed into self-isolation after a staff member at the facility, a woman in her 50s, tested positive for the virus.
The Altona woman, one of five local cases reported on Sunday, tested positive despite receiving her first dose of the coronavirus vaccine on 12 May.
It is possible she was infectious during shifts on Wednesday and Thursday.
Contact tracers are yet to uncover how the worker caught the virus, making her a mystery case .
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Only a third of Arcare Maidstone s 110 staff and 53 of 76 residents have been vaccinated so far, and the federal government has brought forward scheduled vaccinations to Monday.
Covid cases Victoria: "Mystery" aged care case of concern, after five new infections detected on Sunday. No confirmation of lockdown extension as of yet.
Residents at four aged care homes in Melbourne were confined to their rooms on Monday after the federal government allowed staff to once again work across multiple sites.
The Morrison Government has failed to protect vulnerable people living in its residential aged care facilities and the dedicated nurses and personal care.