HMAS Cerberus is the Royal Australian Navy's primary training facility.
A Defence member serving at a Victorian naval base is one of the state's ten new coronavirus cases.
A Department of Defence spokesperson confirmed the member belonged to HMAS Cerberus on the Mornington Peninsula, south-east of Melbourne.
The male member tested positive to the highly infectious Delta variant yesterday, the same day Victorian Premier Daniel Andrews announced Melbourne would enter a snap five-day lockdown.
"The member has been in isolation since yesterday after he was identified as visiting an exposure site identified by the Victorian Department of Health on the weekend," a Defence spokesperson told 9News.
"In response to the positive test, HMAS Cerberus immediately initiated a range of measures to ensure the safety of the base and wider community.
"All training and leaves have been suspended. Living-in personnel have been confined to their accommodation and takeaway meal services are now being provided."
Contact tracers are racing to get on top of the rapidly growing outbreak, sparked when infected removalists travelled from Sydney and another family who travelled to Melbourne broke quarantine rules.
The state recorded six new local COVID-19 cases on the first day of the snap lockdown.
Ten new infections were reported to midnight, but four of those were announced yesterday.
Mr Andrews said Victoria will remain in lockdown until Tuesday night at least.
The five reasons to leave home in Victoria are to care for somebody, work, exercise, buy food and get vaccinated or tested.
Schools are closed and people can only travel 5km from home.