Fair comment, Jack, I shouldn't have just accepted Garland's comment. However, he probably wasn't known by the gaelic spelling. The 2 turn of the century newspaper articles linked by the NZ folk song site refer to him as 'MacKenzie'. A page that I have since come across on the Net has this:

James Mckenzie (or in his native Scottish Gaelic: Seumas MacCoinneach), possibly born in Ross-shire, Scotland, in 1820 was an outlaw who has become one of New Zealand's most enduring folk heroes. The correct spelling of Mckenzie is unclear and he is variously referred to as James, John or Jock. His surname has been spelt as both 'MacKenzie' and 'McKenzie' – the latter being more commonly used. He may also have had at least one alias, John Douglass.

The full article gives a summary of the legend:

Click

--Stewie.