Yes, it is a 10D powered Excelsior Universal. Yes, SOME were sold in Australia rebadged as a Waratah. The Williams family in Sydney who traded under various names, William Bros, and P&R Williams, etc, owned the Waratah name and it is Excelsior coming in through their firm that were rebadged. Excelsior imported by other agencies in other States were not. Various Waratah enthusiasts have tried to pin all this down but with no conspicuous success.
There is a Waratah Yahoo Group but I haven't looked in at it for a long time and there is a Waratah website running out of Sydney but it doesn't seem to have cracked the mystery either.
Your only real way of proving your bike was sold as a Waratah is to find something concrete connecting it to the Williams firm. For example. a dealers plate attached to the bike somewhere or a solid history of it always being a Sydney bike.
You could try very, very, gentle removal of the black paint on the tank and seeing if there is any remnant of the Waratah tank transfer underneath but as that was the sole thing that turns an Excelsior in to a Waratah I wouldn't worry too much. For all restoration and mechanical purposes it is an Excelsior Universal. Only when painting the tank will you need to worry about the Waratah/ Excelsior thing and Excelsior transfers are sooo much easier to find than Waratah.
Pre war Waratah are a completely different kettle of fish. They are definitely an Australian production even if assembled from British components. Post war Excelsiors are just a rebadging operation. There were often good business/commercial reasons for doing this but no real motorcycling relevance.
Tell us what you find