All the rabbits in Australia (200 - 300 million) are the result of 24 rabbits

The current infestation appears to have originated with the release of 24 wild rabbits by Thomas Austin on his property, Barwon Park, near Winchelsea, Victoria, in October 1859 for hunting purposes. While living in England, Austin had been an avid hunter, regularly dedicating his weekends to rabbit shooting. Upon arriving in Australia, which had no native rabbit population, Austin asked his nephew in England to send him 24 grey rabbits, five hares, 72 partridges and some sparrows so that he could continue his hobby in Australia by creating a local population of the species. Many other farms released their rabbits into the wild after Austin. At the time he had stated, "The introduction of a few rabbits could do little harm and might provide a touch of home, in addition to a spot of hunting."

Releasing rabbit-borne diseases has proven somewhat successful in controlling the population of rabbits in Australia. In 1950, after research carried out by Frank Fenner, Myxomatosis was deliberately released into the rabbit population, causing it to drop from an estimated 600 million to around 100 million. Genetic resistance in the remaining rabbits allowed the population to recover to 200-300 million by 1991.

http://en.wikipedia.org/wiki/Rabbits_in_Australia