Mr McGowan announced the borders would be closed indefinitely during a late-night press conference on Thursday.
He said it would be "reckless and irresponsible" to proceed with the planned February 5 reopening given the surge in Omicron COVID-19 cases across the country. WA recorded 11 new infections on Saturday.
But Opposition Leader Mia Davies said people needed more guidance surrounding isolation rules, density rules and the definition of a close contact.
"It's seriously concerning that we have COVID-19 spreading in the community already yet have been left in limbo without a plan," Ms Davies said in a statement.
Federal Health Minister Greg Hunt said the decision to remain closed to the eastern states would hurt families.
"Their ability to meet loved ones, their ability to see beautiful young babies or to be present for weddings, that's all been deferred," he told reporters on Saturday.
Earlier, Mr Hunt doubled down on denials rapid antigen tests have been requisitioned by the Commonwealth, as supplies remain scarce.
Queensland Transport Minister Mark Bailey last week published a letter from a supplier claiming the federal government "commandeered" 34,000 rapid antigen tests originally destined for the state's rail staff.
In a further post on Saturday, a screenshot appears to show a known supplier reached an agreement with the federal health department to provide rapid tests as part of a $26.29 million contract.
"Either the Morrison govt has commandeered the stock using emergency powers, or they've swooped in late with a big money procurement offer of $26.29 million that a supplier couldn't refuse given the massive profit margin offered," Mr Bailey tweeted.
But Mr Hunt slapped down allegations federal authorities had diverted the tests.
"The claims that were made about the Commonwealth are false. They had been referred to the ACCC (Australian Competition & Consumer Commission) because they are false," he told reporters.
"I have confirmed with the Department of Health, the Commonwealth has not received and is not expecting to receive any supply from that supplier until at least February 7."
He said the federal government provided 200,000 rapid antigen tests to Queensland on Saturday, and rejected suggestions it did not plan well enough to meet surging demand amid the Omicron wave.
Opposition Leader Anthony Albanese said the government was warned months ago of the need for widely available RATs.
Australia's two largest states recorded another 48 deaths on Sunday.
Thirty-four more people have died with the virus in NSW and 14 in Victoria.
But daily cases appear to be falling amid the Omicron wave.
Victoria posted 13,091 new COVID-19 infections - its lowest daily figure in almost three weeks - and NSW added 20,324, dropping the state's seven-day average.
Meanwhile, the Northern Territory government has extended lockouts of Alice Springs, the Amoonguna community, Yuendumu and Yuelamu for seven days.
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