It comes as a number of departments and agencies have agreed to allow public servants to continue working from home for January until the rapid spread of the Omicron variant is under control.

In a letter to Public Service Minister Ben Morton on Wednesday afternoon, the Community and Public Sector Union warned the fast-spreading outbreak, in conjunction with testing and contact tracing processes being overwhelmed, meant major workplace transmissions could no longer be ruled out.

Clear and consistent advice to all APS agencies on a number of concerns was needed to critical staff shortages were avoided and public servants were protected.

Mr Morton has resisted pressure to issue an APS-wide direction on working from home arrangements in light of the Omicron outbreak, deferring the decision to individual agencies based on their needs and the health advice.

But union national secretary Melissa Donnelly said this inaction was making the situation more complicated than it needed to be.

"Whilst many APS agencies are facilitating working from home arrangements for their employees, the absence of clear and comprehensive advice on this issue from government is making this harder than it should be and is leading to inconsistent approaches across the APS," she wrote.

In addition to making working from home the default for public servants where possible, the public sector union backed a push to make rapid antigen tests available for staff that had been exposed to the virus in workplace settings.

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Ms Donnelly said positive rapid test results should be considered adequate proof for time taken off from duties due to COVID, in line with recent changes agreed to by national cabinet.

"For front-line employees who need to attend the workplace, rapid antigen tests will form a key part of COVID-19 risk management," she wrote.

"The CPSU supports the view that RATs should be free and accessible for all."

Employees requiring a PCR test should also be given paid leave while awaiting results, she added.

The calls follow a critical shortage of rapid antigen test supplies as COVID cases continue to skyrocket around the country.

States and territories have encouraged Australians to use rapid tests in place of PCR tests after clinics and laboratories became overwhelmed during the holiday period.

But Prime Minister Scott Morrison has pushed back on demands to make the tests free for all, after granting a limited complimentary supply to concession card holders last week

Mr Morrison said about 200 million rapid antigen tests would arrive in Australia over the next four weeks.

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