I think these are two completely different William CALDWELLs.
There is no Greenock or Newcastle in New Zealand, particularly near Feilding. The Greenock where William was from would have been the one near Glasgow in Scotland, and the Newcastle in the north of England. There is a death registered at Newcastle, England in the September qtr of 1907 for a William CALDWELL aged 69, from the FreeBMD website. The Colonist newspaper, like most other NZ newspapers would have had news from "home" at that time, home being the British Isles.
Would suggest you look on the Scotlandspeople website, where you should find him in the Scottish census records. You can get his English death certificate, but it will not give you much information at all.
There is a William CALDWELL buried in the Feilding Cemetery in June 1901. He was 57 and was the one who died at Valley Road, Colyton. He had only been in the district for about a year, having been at Collingwood, which is in the top of the South Island before that.