So, I duly completed the appropriate customs and import forms for sending by courier and off they went. But NZ Post, after taking my money and selling the courier service, sent them on by standard mail. As a result, it cost CAD$600 to collect the parcels at the other end. Ordinary post has a different type of customs clearance and takes no notice of courier documentation. So, they didn't know (or care) that my boxes were not subject to duty. The money has to be paid within 10 days or they send the boxes back to NZ. Once paid, I can submit my import paperwork (which is all in the boxes anyway, if they bothered to open them), complete a form, and wait 12 months for a determination as to whether I get a refund.
NZ Post says they "made a mistake" but because the parcels arrived at their destination they have completed their contract and I am not entitled to a refund of the difference between the courier cost and standard mail fee. They also said they often send by standard instead of courier and people rarely complain.
Nothing to do but apply for the refund in Canada but, damn, I am angry at NZ Post's attitude.