I'm not sure of the procedures and regulations in Australia, but here in the U.S., you would flash your landing light to acknowledge an instruction from ATC. It could be a lost comms situation, where they could or hear or speak to ATC or a partial loss, where they could hear ATC but not speak to them. Additionally, control towers have a light gun that uses a red, green, or white (white only on the ground) to communicate with pilots. So this could have been the case. I have seen landing lights that flash continuously on aircraft however, and I have no idea why. That was on a small private aircraft, not an airliner.