I see that the children of Henry Tasman HOPWOOD have a mother named as Kathleen Winifred and Winifred, and one is Margaret. This may just be a mistake or perhaps she was using an alias. The use of the BYRNES name was perhaps to hide her Chinese background? She may have had children from another relationship and was passing herself off as a widow? I do not see a previous marriage for her at the moment.
Very curious that there is also another marriage for a Winifred AH KIE (Hacker?) to a Thomas BUN IN 1881. Then there is a previous marriage of a Winifred O'CALLAGHAN to a James AH KOY in 1878. I would not be surprised if all of these were your Winifred.