Thanks everyone - it makes a little more sense now...
For me, the most interesting part was that 'my' fellow went to sea at the age of ten. His father was also a sailor, so I imagine he may have gone with him?
Was also amused that, in the later record I found, he spent two years sailing back and forth between Melbourne and Dunedin in 1855 and 1856, and we thought the first of our family arrived in New Zealand in 1968!