Fortunately we in Australia are unlikely to suffer an outbreak of the dreaded H5N1 strain of the Asian Bird Flu. This is because the migrating flocks of water birds which can carry the virus from say Indonesia will probably not survive the flight over. Should a bird survive the trip it would need to infect the local populations which don't migrate and then the virus would need to be transferred to a commercial or backyard flock of chickens before we would have noticeable mortalities.
Its a sad fact that free range chickens are the most likely of the production systems to be the first to be infected. Water birds especially ducks can carry the virus and they are attracted to any standing pools of water in the free range exercise area. The duck poops in the water - the chicken drinks the water and the whole flock quickly becomes infected.
Another infection route is when egg or chicken meat producers draw water direct from a dam or river and supply it to their flocks without disinfecting it first. To obtain quality certification producers need to have an effective disinfection system in place and it must be maintained. These regulations don't apply to the neighbor who keeps a few chooks for eggs for himself and a few friends.