With the departure of many [Muslim] cameleers in the early 20th century, and the introduction of motorised transportation in the 1920s and 1930s, some cameleers released their camels into the wild…..[By] 2008, the number of feral camels was estimated to be more than one million, with the capability of doubling in number every 8 to 10 years. (my emphasis)

Source Australian feral camel – Wikipedia