How hot / humid does it have to be for cycle touring to become unenjoyable?
I was out in 28C and 50% RH yesterday, not touring, but comparable: a century (160km) at a fast touring pace, laden by the standards of a day ride, and with a brief swim break in the afternoon.  It was hard work (much harder than it would be 10 degrees cooler), but still enjoyable.  You have to plan on being able to obtain water. I got through about 4 litres, and have reached 9 litres in the past on hot 200km rides.  Especially for multi-day riding, like touring, will your appetite survive the heat? I tend not to eat enough on a long day's ride, if it's over about 25C by lunchtime, though if you can buy ice cream...
Road conditions matter - dropping into a wooded valley from an exposed hillside is a great relief.  How likely is shade?
If you can avoid the hottest part of the day, that helps, but many areas lose heat slowly, so avoiding the heat means doing most of your riding in the morning, resting or sightseeing in the afternoon, and maybe doing a little in the evening.  But does this fit with your target daily distance?  Is there a shady (and pleasant, with food/water) place to rest out the heat?
Clothing is tricky.  Clothing for sun protection keeps the wind off, so even if it didn't make you hotter standing still, it would going along.  Arranging clothing to allow more airflow means it flaps in the wind and makes you work harder.
Guessing at your route as Sydney-&gt;Brisbane (but not along the coast), and quickly checking the map, the road passes through plenty of towns, so places to stop even if most are small, however street view suggests you'd be riding in unbroken sun for long periods - I've been known to stop and drink under every rare tree in that case.