In the Bible, there is a handwritten entry about a Thomas McCreath who went to NZ on board the "Sir William Eyre" in early 1860s, arriving at Dunedin I believe. He may have been born in 1840.
.. haven't as yet found the name of Thomas McCREATH (nor indeed anyone named "Thomas") amongst passengers listed for the voyage you mention. That is not of course to say that Thos. McCREATH did not travel on this particular ship.
But of course the presence of a "Thomas McCREATH" in the area of Southland NZ (including his death in that location, a marriage in 1865, etc. and perhaps his unusual surname ), certainly suggests he is the same man you mention.
The "Sir William Eyre" (yes, it was an horrendous journey), turned back in December 1862 to Rothesay with its large group of emigrants who'd been destined for Southland province in New Zealand. When later the voyage resumed, the ship was forced to put into Cape of Good Hope for some time. She finally arrived at the Port of Bluff (at the bottom of the South Island, NZ) around 8 April 1863 - a journey of some four and a half months.
Of the passenger lists I've located to date, all seem to have been compiled from resources other than an actual passenger list. i.e. Newspaper reports, old settler's records, jubilee lists.
That I haven't found the name of Thomas McCREATH (or variant or misspelling of name) might mean that he was not perhaps "an actual immigrant" to Southland" (persons who had all or a portion of fare paid to the Colony under an official Immigration scheme). Could he have been a member of crew of this ship ? Or a passenger paying full fare ? (Names of full-fare passengers - or persons having their fare paid by someone already living in NZ, were sometimes not always recorded along with the names of "official immigrants".)
I'll keep looking for additional information.
~ Lu
