Western Australia will halve its international arrivals cap and tighten the New South Wales border after recording its first known Omicron cases in hotel quarantine.
Two returned overseas travellers are confirmed to have the Omicron strain of the virus and are being monitored in state-run hotels.
WA currently receives 530 international arrivals per week but the government will halve the cap from next Thursday.
It comes after a number of breaches involving ventilation issues at Perth's quarantine hotels, two of which prompted short lockdowns earlier this year.
"Halving the arrivals cap will ensure capacity in hotel quarantine and safeguard models are maintained given the heightened Omicron risk and addition of NSW approved travellers in hotel quarantine," a state government spokesman said on Wednesday.
From 12.01am on Saturday, NSW will join Victoria in moving from high risk to extreme risk under WA's controlled border regime.
Travel will not be permitted from NSW except under extraordinary circumstances, with approved travellers required to return a negative test, be double-vaccinated and undergo 14 days' hotel quarantine.
The daily infection rate in NSW has surged to 1360 amid warnings the state could have 25,000 cases per day by the end of next month.
Premier Mark McGowan urged West Australians in NSW with an approved G2G pass to return home immediately before the tougher restrictions kicked in.
"The situation in NSW is very concerning and the cases have rapidly risen in the past few days so we need to do everything we can to keep Western Australia safe, while we get WA's vaccination rate up to 90 per cent and remain free of COVID in the community," he said.
"I implore everyone to get vaccinated and ensure they get their third doses when eligible."
WA is due to reopen its borders on 5 February but the government has vowed it will take harsh measures in the meantime to keep the virus out.
About 80 per cent of West Australians aged 12 and over are fully vaccinated.
Authorities continue to trace the movements of three truck drivers who tested positive in South Australia after spending time in WA last week.
The first two men, aged in their 30s and both fully vaccinated, entered WA together on December 9 after returning negative rapid antigen tests at the SA border.
They visited a number of sites in Perth's eastern suburbs, including a supermarket and a pizza outlet, before departing early on 11 December.
PCR tests undertaken the following day as the men travelled through SA to NSW came back positive.
A third driver, aged in his 20s and also fully vaccinated, also tested positive after spending time in WA between 9 and 10 December.
Authorities are investigating whether he was infectious during that time and said he is not linked to the other two infected drivers.
WA Health has so far identified nine exposure sites and 55 contacts.
