“If, for example, just NSW rescinded their orders, a flight from Brisbane to Melbourne would have no mask requirements whilst flying over NSW, but still required when over QLD and VIC.”
i don’t think the airspace you are currently in determines jurisdiction. But I’ve always wondered: suppose you are pax on a US registered airplane but leased to say a Spanish carrier and you commit a serious offence eg a murder. Which law applies a) while at the gate in sydney, b) taxying out, c) airborne over Australia, d) enroute over pacific. Not thinking of doing it but would like to know anyway