Surely they couldn't claim in any measure to be 'Australian' if they were fully imported.
There's the rub: Williams Bros. most often claimed the Waratah to be "British"! This doesn't necessarily mean that they were actually made in Britain - for example Invincible JAP liked to mention the word "British" a lot, despite being made in Australia - but in the case of Waratah I'm now wondering just how many, if any, bikes were built or assembled in Australia.
There is a large Wikipedia page on Waratah, compiled by a number of authors:
https://en.wikipedia.org/wiki/Waratah_motorcycles with lots of references to original articles. Unfortunately the authors don't seem to have followed through with the info in some of these.
Over here in South Australia, we had Elliott Bros producing Villiers-engined bikes in reasonable numbers through the 1920s. I happen to know they assembled them in their factory, in part because I have a box of left-over lugs in the shed that came from Elliott when they closed! But is there any evidence of Williams Bros actually making anything?
Let's explore the "all Waratahs are badged Excelsiors" idea until we come up with something that is certainly not Excelsior. Then we can think about where that came from!
I see there is a Waratah Facebook page, which neither Google nor I can see. Perhaps they know the proper story?
Cheers
Leon