Andrew, that 3000 miles gets you to the Marquesas where there will be a clinic but I am not sure about a pharmacy. You can make a shorter passage via Easter Island but you would have to see about insulin availability at Easter (it is a pretty advanced place), Pitcairn (obviously self-contained but whether they would have insulin or whether you could arrange delivery on the supply ship I don't know). Next stop would be Mangareva in the Gambier Islands where if visited the quite good, small clinic. i am sure they would have insulin but don't know if they well sell to visitors. You could arrange to have some flown in from Tahiti. Planes come in every four or five days. Once you got to Tahiti I think it would not be a problem. I can't comment on the specifics going to New Zealand, but with the normal stops going to Brisbane you would be fine.