Such clause does not exist. If there is a special requirement for transit it is metioned within the entry rules of that country and best looked up on timatic.
However, OP had a ticket to SIN and not to Australia with EK/QF. At departure the airline requires proof for entry allowence at destination of this travel. It doesn't matter to the airline how the pax will proceed onwards, transit,layover, walking, swimming or flying on an other ticket.
Therefore the only solution would have been to book a through ticket to Australia. It does not help to book another ticket SIN-MEL to shorten the stopover into a transit layover because the first trip still ends in SIN.