I am always surprised to discover how little overseas birders know about Australia. I get a bit tired of it sometimes. Fair dinkum, people seem to expect to see Red Kangaroos hopping down Bourke Street, dodging the trams and slipping into Young and Jackson's for a quick beer. But I'm off-topic. back to hummingbirds.
Like so many other northern hemisphere creatures, Ruby-throated Hummingbirds were introduced to Australia in the mid 19th century, initially around the Sydney area, but with secondary introductions in Queensland and (for reasons that escape me entirely) near Hobart. The NSW and Qld introductions were successful, the Tasmanian one obviously failed (far too cold down there).
Since that time hummers have spread rapidly, reaching the Murray River around 1880, spreading into South Australia just before the turn of the century, and first being reported from the hills outside Perth immediately after the First World War.
As introductions go, hummers are middle of the range, having proved neither as harmful as the Red Fox, the European Rabbit, and the Common Starling, nor as benign as the Spotted Turtle-dove and the Goldfinch. Although common, they cause little damage to crops, and do not compete with native birds for nesting sites. The main problem resulting from their introduction has been the impact on the threatened Australian Bustard, which seems not to have adapted well to predation by the large flocks of hungry migrating Ruby-throats. An active captive breeding program, coupled with reintroduction of the bustard to reserves with hummer-proof fencing seems to be dealing with the issue.