http://www.environment.gov.au/biodiversity/invasive/publications/cat/pubs/cat.pdf
The National Parks & Wildlife Service estimate that there are over 400,000 feral cats in New South Wales, and as many as 12,000,000 (twelve million) scattered throughout Australia!
NB this does not include domestic cats allowed to roam.
Cats are known to kill and eat more than 200 separate native species, including birds, mammals, marsupials, reptiles, frogs, and invertebrates.
Cats eat around five percent of their body weight each day. This may include native animals, introduced species (such as mice, rats, rabbits and introduced birds), and scraps and carrion around urban centres. This is the equivalent of one bush rat per day. One cat caught near Roxby Downs was found to have eaten almost thirty small reptiles, probably in the one day.
Twelve million feral cats killing animals every day. Even if only one animal in ten was native, it would be reasonable to assume that cats are responsible for killing hundreds of millions of individual native animals every year.
Feral cats are known to carry and spread disease such as toxoplasmosis which can be fatal for several native species, as well as causing sickness in other natives, livestock, and even humans.
In 1992, at a cattle station in the South Western Australian outback Professor J Pettigrew of the Universtiy of Queensland shot 175 ferals in a 10 sq km area. The army shot a further 400 in three days yet a few weeks later they returned to shoot a further 200. According to Professor Pettigrew cats were pouring into the vacuum created by the extermination program.
http://library.thinkquest.org/03oct/00128/en/cats/effect_light.htm