Three of the new infections are linked to a Delta outbreak in Perth that was triggered by an infected French backpacker.
That cluster has grown to 21 cases after the unvaccinated 25-year-old male index case travelled from Queensland in early December.
A Hyatt Hotel quarantine worker has also tested positive, Acting Premier Roger Cook told reporters on Wednesday.
"An investigation is underway into this case," he said.
The infection was detected during a routine test and more exposure sites are expected.
The fifth local case is a close contact of an interstate arrival who was quarantined at a home.
Two of the three new backpacker-linked infections were likely to be infectious in the community and more exposure sites are expected to emerge.
One of the cases was announced late Tuesday with the Lucky Shag Waterfront Bar and Aviary Bar listed as exposure sites.
There were also 11 cases found among returned interstate and international travellers and air crew.
The Lucky Shag Waterfront Bar confirmed the case was a staff member.
It comes as authorities confirm an infected quarantine-hotel security guard who tested positive on Sunday while working at the Pan Pacific has the Omicron variant.
"While this is serious, in encouraging news it does not appear that there has been any further spread," Mr Cook said.
Fifteen close contacts have been identified, with 14 testing negative and one test pending.
Investigations continue into how the infection happened.
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