Sorry, I don't have any Williams in the branches from my Great Great Grandfather Captain David Thompson who died in 1900. What I have found was that he had 2 Nephews who I know nothing about or their line that attended David's funeral, going by the Sydney Morning Herald at the time. The Nephews names were William and Alexander Thompson. I would believe a brother of David must have come to Australia as well, the father of his Nephews.
I don't know if you see any possibilities in this one.
Regards, Jim Thompson