"This is the most likely event to have community spread of the virus in WA that we are unable to crush and kill," he said. "That is a fact."
WA health authorities are now upgrading the contact status for the Perth Mess Hall event, which was attended by about 400 people, so that those who attended must get tested and self-quarantine for 14 days.
It means those people will be in quarantine until January 3 – missing Christmas and New Year's Eve celebrations.
The 25-year-old unvaccinated Frenchman, who has been in Australia since before the pandemic started, arrived in WA from Queensland on December 12 – just one hour before the border restrictions were strengthened.
He felt sick on Sunday and got tested on Wednesday, with the "strong positive" result coming through on Thursday.
In response to the new outbreak, McGowan announced that all licensed hospitality venues would only be allowed seated service from 6pm on Friday. In addition, the use of masks will be mandatory in all public indoor settings in the Perth and Peel regions until 6am on Tuesday.
Large public events, such as music festivals, have been cancelled, while nightclubs have also closed and all dancing is banned except for weddings.
"The measures we're putting in place here for five cases are as tough as the measures in NSW for 5000 cases per day," McGowan said. "But we are in a bit of a twilight zone. We don't know how much spread is out there so we're trying to work out what it is."
McGowan further announced that from 12.01am on Christmas Day, South Australia and Queensland would be reclassified from "medium risk" to "high risk" jurisdictions under WA's tough controlled border measures.
WA is expected to reopen its borders on February 5 but McGowan has repeatedly said that date could change.
"If we have major community spread of the virus across the community, it may well be that February 5 becomes redundant," he said on Friday.
Of the eligible population in WA, 83.1 per cent are fully vaccinated and 90.6 per cent have had their first dose.
