Any time a government makes a change to a country's circulating currency, it really gets the collector market going. Take a look at the times when this has happened in Australia: the change to decimal currency in 1966; the introduction of the $1 & $2 coins (1984 & 1988); the removal of 1¢ & 2¢ pieces (1991); the introduction of polymer banknotes (1988 to 1996), the list goes on. Each one of these events had an impact on the market for coins & notes in Australia, pushing it upward each time.
The RBNZ's recent decision to change the size of a range of New Zealand's circulating coinage has apparently resulted in "collectors" going mad buying "rare" decimal coins for big money. If you take a look at activity on the NZ auction site trademe though, most of the sales (and attempted sales) of ordinary coins for what seem to be massive prices are by non-collectors or novice collectors.
Nothing unusual here (we see it on eBay Australia every day of the week for ordinary pre decimal notes etc), but it will be interesting to see if the NZ market reacts over the long term to this change, and if so which coins are the best ones to put away.