In summer where I live it gets hot
Typical temps are around 36 degrees (97 Fahrenheit). On extreme days it can get to 42 (108 Fahrenheit)
When it's 36 my bunnies may remain outside but they have a large covered area under a trampoline with the pen extending beyond it by a fair margin
There are a couple of overhanging trees as well
At this temperate range, I will go out about ever 45 mins - 1 hour and squirt them down and all the surrounding area so that the dirt is damp and cool. They also get a spray as well although it's often does as a mist or on a shower setting and I make it like it rains and will often shower the tree and let the water run off the leaves onto my bunnies. I will also squirt down the trampoline which is semi-permeable, so the water seeps through onto the ground underneath. They also have a separate cage that they can run into for cover/protection. They have hollowed out an area underneath it to access the cool dirt
One of my bunnies spent some time in the wild before we got him, so he's water smart and takes cover. The other sits there letting the 'rain' fall on him before he decides he will move
I check on them all the time to make sure they don't overheat. It's cooler out there in the pen then elsewhere with the shade etc
If it goes above 35 - 36 C, then I will bring them in (after they are dry) to the air conditioned house
Even at 35-36 C they will often run around, which I know they will only do if they are not too hot
They have plenty of shade (under the trampoline as well a tree shade), plenty of cool dirt to lay in, plenty of water to drink. If it gets too hot, then I bring them in
Main thing is I think to keep an eye on them at all times, keep them cool and when it's too hot, bring them in (which they hate no matter how hot it is outside, there's something they love about being free to frolic outside as a bunny should)
Now in only stupid Australia would allow myxomatosis vaccine it might be ok for bunnies