Perhaps you have access to more recent information, but according to everything I can find, there is no myxo vaccine approved for use in Australia. Calicivirus, yes (and some sources even recommend this vaccine twice a year), but not myxomatosis.
https://www.rspcavic.org/health-and-behaviour/rabbits/rabbit-vaccination
So basically, if there are wild rabbits in the area, and mosquitoes and/or fleas, a pet rabbit is at risk for myxomatosis. The only measures that a pet owner can legally take to protect a pet rabbit from myxo is to create a mosquito-proof as well as predator-proof enclosure using mosquito netting or screens. Otherwise, you have a choice of either keep your pet inside, or only let it out at times when mosquitoes aren't active.
How do rabbits do in extreme heat? Well, it depends. Anything above 30°C is a danger zone for rabbits. Fans and frozen soda bottles are old stand-by's, but they may not be enough in real scorching heat. Someone in our southwest said they built underground boxes for the rabbits to shelter in during the day; soil is a very good insulator.
