A few clues about how long you had them, how they were planted, and follow-up care could help us a lot.

Here's the Aussie rhody society -- you should find lots of help there:

http://members.iimetro.com.au/~mirra/australi.html

And/or have a chat with your neighbor

In any part of AU reaching 30C regularly, you need to grow the right varieties. A lot of the tougher ones down there are descended from R. arboreum because most forms of it are heat and humidity tolerant, and winter hardiness is obviously not an issue. For example R. 'Bibiana' grows in the hills around Atlanta, Georgia, USA and should grow very well for you too, assuming the soil structure and ph are OK. 'Lord Roberts' is supposedly very common in parts of NZ & AU but the flowers are ugly by comparison.