Originally Posted by
findark
I'm not aware of Schengen countries requiring proof of onward travel. Can't remember if I've ever done a one-way flight but I don't think they care - TIMATIC does not show a requirement for onward travel. Possible a round-trip ticket with a longer duration could raise flags. But I think an employee seeing a flight to a Schengen country will just look at your US passport and approve the docs.
I'm not sure OP has a US passport -- which could be the source of their concern... The rest of your post is 100% spot on -- OP -- Aussie passports can also enter the EU visa free -- I've found that for foreign passport holders many times you just can't check in ahead of time -- this happens to my (Japanese) wife all the time... Try not to worry...