Hi Woodsy, I'm in Ballarat so not a generally HOT area, however I have similar problems with my 1KZ when it is a hot day and I am on highway at 100 kph, the temperature gauge gets past 3/4.... I don't keep the AC on long enough to see if it is still functioning correct as i figure this is more strain on whatever is struggling... once i slow down to about 80 kph the temperature gauge settles down.
I visited a local radiator specialist recently and he reckons it is a dirty/ partially blocked radiator.
I am planning to put a new radiator in in the new year and am hoping this may resolve the problem for me, however this did not work for you
Keen to see if anyone else has had similar problems and/ or solutions.
I've ran out of brick walls to bang my head against Rob hope you have luck keep me posted you never know the new radiator I have may be faulty.