
Likely to be aluminium phosphide (possible brand name Phostoxin) which is a non-anti-coagulant gas producing tablet style poison. Its applied IN the burrows of rats/rabbits/moles and produces gas when it gets damp (which it does from the air).
That being so, most animals die IN the burrows so you shouldn't come across loads of dead bunnies, but it is horrid stuff and I would stay the heck away from that area of land for several weeks to be sure that the stuff has broken down.
There is this thread from a couple of years back also -
http://www.champdogsforum.co.uk/board/topic/109856.html