Hi Alistair. If William Royse of Dunedin is your great grandfather then you must be descended from one of his sons. Which of the sons was your grandfather?
The originator of this thread, Barbara from Australia, did an enormous amount of work on the family genealogy and that is available for distribution.
Send me an email to: rennes(at)xtra.co.nz and we can correspond further. As a matter of interest which country do you live in?
Best regards Paul Royse