Thanks shume and Dundee
Given the so aptly described “Bermuda Triangle” conditions on the goldfields, and no trace of William CURNOW in the civil death records in Vic or SA, my hunch is that he died in Bendigo before the rest of the family moved to SA about 1860. The death of his son-in-law John THOMAS in Jul 1857 in Bendigo is well documented so it is more likely that William died earlier than that, ie 4 Jan 1853 - Jul 1857
According to the 1841 and 1851 English census returns William CURNOW was a shoemaker / cordwainer. I wonder if he worked in that trade on the goldfields