Panchomaha, if you can, freeze some water bottles, wrap them in a towel, and put them in with the bunny, so they can choose to lie up against the cold bottle, or not. You can also wet their ears with cold water.
I too live in the hot Central Valley, and when our AC broke down, these things helped a lot. Also hanging damp towels, frozen if possible, over their cage (not covering completely), and blowing a fan on that.
You can also put a few icecubes in their drinking water.