I think as vaccines are now available and widespread there is a better argument for Section 92. Flight Centre might have a chance at declaring WA travel bans unlawful in the High Court.
Section 92 of the Constitution of Australia states that "trade, commerce, and intercourse among the States, whether by means of internal carriage or ocean navigation, shall be absolutely free."
This is subject to permissible regulation which might take the form "of excluding from passage across the frontier of a State creatures or things calculated to injure its citizens", but the severity and need for such measures must still be assessed. (The Nationwide News Pty Ltd v Wills ruling)
During a pandemic, therefore, the Premier had the emergency power to close the borders to a disease that could cause injury. But McGowan can't hold out too long. The High Court may look at the severity of risk, reassess McGowan's measures and determine that the measures are now economic (remember when McGowan admitted he was stopping SA and NT travel so WA money got spent in the state?) or political, and not health related.
If that's the case then WA travel bans stop. Nothing McGowan can do. Free travel between states is a core part of the Australian Constitution.
