“Authorities”?? It’s a political decision, and has been justified in round terms, and all there are is media bytes:
Mr Morrison said there was not yet “considerable clinical evidence that tells us transmission is preventable” after the jab.
Prime Minister Scott Morrison says there would be at least 1000 coronavirus cases in Australia every week if the international borders were to open, even when vulnerable people are vaccinated
I don’t think that the borders are remaining closed for a ‘specific’ reason: ScoMo isn’t a ‘specific’ kind of guy.