The Perth metropolitan area, as well the Peel and South West regions, have all been declared red zones under Victoria's traffic light permit system.
Anyone who has been in one of the zones since January 25 cannot enter Victoria without an exception, exemption or work permit.
The announcement of the closure was made Sunday night, a few hours after WA announced its snap lockdown, which came after a quarantine hotel security guard attended more than a dozen venues while infected with COVID-19.
The lockdown will continue until 6 pm on Friday.
The news has come as a shock, after WA enjoyed almost 10 months with no coronavirus cases in the community.
Health authorities believe the man in his 20s, who worked at the Sheraton Four Points hotel in Perth's CBD, has likely contracted the highly contagious UK variant of COVID-19, as he was working on the same floor as a person infected with this strain.
The man returned a positive test result overnight Saturday after he last worked at the hotel on January 27.
He started experiencing symptoms the following day and called in sick before being tested.
He returned three negative tests between January 15 and 23 - it is believed he was first infectious from January 26.
The man's three housemates have returned negative test results but will need to complete a 14-day quarantine, while all other close contacts have been ordered to self isolate.
This comes as Victoria recorded zero new locally acquired cases of COVID-19 for the 26th day in a row.
State health authorities are continuing investigations into a weak positive coronavirus test result detected Saturday night.
The man in his 30s received the result following a negative test taken two years earlier.
"Investigations and re-testing have been undertaken on an indeterminate COVID-19 test result detected late (Saturday) night," DHHS said in a statement.
"Out of an abundance of caution, public health actions have been taken, including isolating and testing all close contacts of the case.
"Initial repeat follow-up tests of the case and those of close contacts are negative but further investigations are ongoing."
Meanwhile COVID-19 fragments have been found in wastewater samples at six Victorian locations since Friday, including Castlemaine, Cowes and Pakenham, as well as Leongatha, Gisborne and Hamilton.
Anyone who has been in those locations in recent days is urged to get tested if they display even mild symptoms.
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