Movement of people and goods across state borders in Australia is guaranteed by the Constitution. Section 92 of the Commonwealth of Australia Constitution Act 1900 (Imp) says:
trade, commerce, and intercourse among the States, whether by means of internal carriage or ocean navigation, shall be absolutely free.
“Intercourse among the States” in this context, means the movement of people, goods and communications across state boundaries. Shutting state borders is to fracture the federated Commonwealth.
Also a state cannot exclude people from entering it because it has some objection to them, such as their character or behaviour. For example, shortly after federation, NSW enacted the Influx of Criminals Prevention Act 1903 to prevent convicted criminals from other states entering New South Wales. It tried to use the act to prevent John Benson, a convicted vagrant, of entering the state.
But the High Court struck down the law because it prevented freedom of interstate movement in breach of section 92 of the Commonwealth Constitution.