The new case also attended an busy event at Perth Mess Hall in Northbridge a week ago and was staying at a hostel linked to a previous case, WA Health said on Sunday.
It brings the total number of locally acquired cases in the current outbreak to eight.
All are assumed to be the Delta variant, after the 25-year-old index case - the first identified in the group of cases - was confirmed to have had that strain of COVID-19.
The unvaccinated backpacker visited nightclubs, shopping centres, restaurants and a cinema across multiple Perth suburbs after arriving in WA on December 12.
The newly detected case tested positive while in quarantine and is not considered to have been contagious while in the community.
An international traveller has also tested positive in hotel quarantine overnight.
There are now 18 active confirmed cases in WA, 15 in quarantine hotels and three in self-quarantine.
Meanwhile, police have charged a 25-year-old French tourist in hotel quarantine after he allegedly left his room, threatened security staff, resisted arrest and hurled abuse on Christmas Day.
He was charged with failing to comply with a health direction, threatening to destroy, damage, endanger and harm property, and obstructing public officers.
The man charged, who is not the same person as the index case, had been staying at the Old Fire Station Backpackers in Fremantle and was deemed a close contact on Christmas Eve.
The man is scheduled to appear at Perth Magistrates Court on Sunday.
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