I'm with Rami, I wouldn't be comfortable using this directly on rabbits unless I could find scientific proof that it was safe.
There are other options available. For fleas, you can use one of the rabbit-approved spot treatments that are given every month. In the US, the safe ones for rabbits are Revolution and Advantage. You have to go to a vet to get them. All other furry pets, especially cats and dogs which go outside, should also be treated regularly to prevent them from bringing infections fleas inside the house.
In the UK, where keeping rabbits outside is not only common but often recommended by many rescues, there is a product called Rear Guard that is supposed to deter flies, with the aim at prevent flystrike/maggot infestation. It would be worth checking to see if it can be found elsewhere too, as I don't think treatments like Revolution and Advantage do anything for flies.