I'd like to add a little to this debate, if I may.
I'm running off memory here, so I can't give you any difinative dates.
The Crown of Thorns starfish was discovered off the coast of Australia a few (or more) years ago, it is believed that they were contained in bilge water in a ships hull which was discharged in a port here. This was all it took for them to get a foothold.
Unfortunately for us that's how most of our more recent problems have occurred (i.e. fire ants), the pest hitchhikes on or in something that is transported from somewhere else in the world, hops out when it gets to Australia and settles in for the long haul!
Our quarantine laws are among the best in the world, but things still slip through occassionaly.