Mate, how have you gone in Rangiora / Christchurch finding well formulated HOAT?....I had some issues here (just outside Sydney) and went to Valvoline to find a supplier of Xerex GO-5. I think it was worth the effort. From lots of reading about these overheating issues I have the come to the conclusion that the correct fluid specs is utterly critical.....not so much the mix ratio - and despite the good water you have you MUST use at least demineralised water - preferrably distilled.
This vehicle has to run HOAT (as papindigo said) and in my view it has to be specifically compliant with Chrysler MS9769.
Is the water in Rangiora a bit sulfury
Had a chat with the guy who sold the Xerex to me (who is also an Agricultural Engineer so we had a good sit down yarn) and he was quite concerned about the shortcuts blokes are taking with their fluids. We both reckoned no substitutions. Mine is a 2006 so it will be a bit different....I assume yours is a 2003 2.8 or is it a 2.5?
Don't disagree with any of the posts so far especially the idea of looking for localised hot-spots with a laser thermom. However, you seem to be zeroing in on it with the viscous heater comment but is there any way you can get hold of a code. As you have a 2003 model you might be able to do the key code recall. It might be worth a read through FSM to work out when it should be running.
Hey what a great part of the world you live in. I spent 6 months in NZ 30 years ago. That area heading west from Christchurch is heaven. I know I said about the sulfur but I can't see it with the volume that flows down the Rakaia and other rivers in that area. Still wouldn't put 'em in the Jeep though.

There are ways to get the fluid if that contributes to the issue - we can discuss later if needed.
Cheers