You'd know better than me, but if you bought two tickets (i.e., non-connecting), don't you need to clear customs at the first port? If that's the case and someone arrived from Europe in Singapore, it's a mandatory 14 day quarantine before they can catch the second flight. If it's a connecting flight, the travel data would be available to Australian Customs.
Anyway, I am sure govts are alive to this risk - the folk that flew in from NZ yesterday were apparently subject to an intensive screening process which requires verification that the traveller was in the 'bubbled' country for 14 days prior to departure. So I think the risk can be mitigated, particularly where there is a formal bi-lateral bubble arrangement.