I'm also in Australia, and yes, there are several jurisdictions here that stipulate that P Platers must not have ANY trace of alcohol in their blood. In fact, I recently watched one of those reality police shows where a teenager was booked because he had traces of alcohol in his blood. It turned out that his girlfriend's mother had given him a few rum balls, which (obviously) contained traces of rum! Luckily, he was let off with a warning.
To me, a lot depends on what type of food it it. For example, a dessert traditionally made with alcohol, such as trifle, tiramasu, or rum balls, I'd expect the guest to have a pretty good idea that alcohol might be an ingredient.
For things such as sauces and marinade, these can just as often be made without alcohol as without. So I think the onus is on the host to inform guests if there's any alcohol used in things like that. Especially if they know (or ought to know) that that particular guest cannot consume alcohol, for whatever reason.