Hi Matthew
From your last post, are you saying that potentially your Alexander's parents, Alexander and Janet (Masterton) also headed off to Australia? Certainly, from what you have found, youngest son David, whom we had in the family's 1851 census entry, also seems to have ended up there.
Monica
added: Possible migration for David?:
David Turnbull, b. 1845 arrived at Sydney, New South Wales on 18 Aug 1864, ship 'Yarra'.