I have travelled a few times from Hamilton to Sydney in the past under Air NZ and prices were around $1300 for two (an average of $325) compared to PacBlue offering $199.
Rotorua will be slightly more expensive than Hamilton (at $209 I think) but my point is that Hamilton, Coromandel, Tauranga, Raglan, and other surrounding regions now have a much more compelling reason to fly Hamilton than Auckland (price parity, cheaper parking as well as not having to travel to Auckland, and a quicker processing time). Much of Hamilton's travellers went through Auckland, so loyalty to the region was a big reason for passenger numbers falling from HLZ in the midst of high fares by Air NZ from HLZ.
My guess is that Air NZ took a gamble that if they pull out of Hamilton, thus potentially increasing their loadings out of Auckland (and therefore greater efficiency), that no other carrier would take their place in Hamilton. Problem was, someone did, and Air NZ made such a mess of themselves when they left Hamilton that no one would welcome them back, so what better than to drop in to Rotorua and try to compete with Pacific Blue from next door.
Good on Rotorua for securing internation services, but I am pretty confident that Rotorua cannot survive, surely. If Air NZ claim Hamilton did not have the passenger numbers to warrant international services, how is Rotorua any different? The catchment area covers the same area by both airport's claims that they are both the "gateway to the central NI". I can only see it as a strategic move in anticipation of shunting Pacific Blue out and then Air NZ will withdrawal from Rotorua because the catchment still fails to provide number that warrant services.