Perth, Peel and the southwest region - around 80 per cent of the state's population - was sent into a full five-day lockdown in response to the outbreak from 6pm local time on Sunday.
"We are told the guard was working on the same floor as a positive United Kingdom variant case".
On the circumstances of the case, he said: "It appears possible that this new positive case has the highly transmissible new United Kingdom variant".
Acting Premier Steven Miles said border restrictions were being "ramped up to protect Queenslanders".
The Government advice on Australia now states: "If you are arriving in the United Kingdom from Australia on or after 4am on 18 January you will need to self-isolate on your arrival unless you have a valid exemption".
Mr McGowan, in his news press conference, told that the decision might come as a shock to people of Western Australia.
"We are strongly encouraging that everyone in this area who is from another WA region, stay here". I urge the community to act calmly.
"This week is absolutely crucial that we stay home, maintain physical distancing and personal hygiene and get tested if you have symptoms", McGowan says.
While the government has allowed travelling only for essentials things or in time of need, wearing masks has been made compulsory. Except for essential work, healthcare, food shopping, or exercise.
The Australian city of Perth was ordered into lockdown on Sunday after a security guard working in hotel quarantine tested positive for COVID-19, ending the country's longest coronavirus-free run.
Schools, restaurants, bars, theatres and gyms will be closed during the period.
Visits to health facilities are banned and weddings are cancelled.
Australia has been strong and upfront to people about the inconvenience caused but can not help contain the virus.
He also urged people not to panic-buy, saying the transport of essential goods into the region would continue.