Thanks for the awm clue. I'll follow it up.
True about the 18!
From her death certificate, that was over 26y, from m1 at 17y to m2 at 43y.
just a note: a variant of Sunnerdale could be SUNNERDAHL
Also, it's quite possible her grave was originally marked, but that her headstone was one of the victims of the Brisbane City Council's beautification scheme in 1975 - 76. According to the Friends of South Brisbane Cemetery's 2008 book by Tracey Olivieri about the cemetery's early yrs, about 100 headstones were removed then & used for purposes like infilling gullies. The council did evidently try to redress this later.
Also, Thomas's headstone, if it existed, could have been used for roadfill. Truth Nov 17 1907 evidently had an article about this. Other sites about it are brisbane history, chapel hill site about Paddington cemetery and the Brisbane city Council site about historic cemeteries: Paddington. Again, the council has tried to redress this. He could lie under what was Lang Park, now Suncorp Stadium. Lang Park was named after the person who arranged for Thomas's siblings to come out on the Chaseley & Fortitude. Brisbane's Fortitude Valley, where, from her death certificate, Harriet lived with her 3rd husband, was named after that ship.