Not sure you really want to know, Shelleywba.
Refer to question on fly longevity (
http://www.theanswerbank.co.uk/Animals_and_Nature/Question57 039.html), and think of a rabbit's behind as the food source. Ugh, ugh and double ugh.
The main thing is to make sure your bunnies are clean and dry, and don't get too hot. Ours always had plenty of good natural food such as vegetable peelings and hay, and we never had any problems. I think it's squishy droppings which tend to give the flies somewhere to start by sticking to the fur.
A friend of mine had a human bot fly last year, but I don't think even that was as bad...