Easy they could fly Auckland- Napier then Napier to Christchurch they already are in talks with Hawkes bay airport.
The reality is more like Hawkes Bay airport (and others) are chasing the likes of Jetstar, Jetstar aren't chasing them. I'd say the thought of and invitation to go to some of theses places is probably more of a distraction rather than a reality for Jetstar.
The only way many of theses regional airports will get the likes of Jetstar, if ever, is to offer big incentives. These rarely work as they found out in HN with Virgin. The irony is they offer incentives to get a new player to provide cheaper prices while all the time never offering any help to the incumbent to help them provide cheaper flights.
Christchurch to Hamilton is another even Christchurch to Tauranga. Wellington to Dunedin is a route they could go for as well. Still no options you think?
Have you checked to see how many flights a day there are between TG and CH and HN and CH? Probably hardly worth an A320.
It all comes back to the population base and the need for people to travel from/to theses places. Also regularity of service is important several flights a day is preferable to one flight a day.