While ships are in port loading and unloading cargo, they need to pump large volumes of ballast water in and out of their ballast tanks so they remain stable. Any tiny creatures living in the water will get a free ride.
There are masses of microscopic marine life in the ocean eg bacteria, invertebrates, algae and the planktonic stages of marine life cycles like eggs and larvae. Most of this life is small enough to pass through the ballast water filters, pipes and pumps.
As Australia is a continent surrounded by water nearly all our international trade depends on ships and the ocean. Approximately 12000 international vessels arrive in Australian ports each year.
While ships are travelling over the ocean and providing it is safe to do so, ships replace the ballast water from their last port with ocean seawater. The ocean seawater does not have coastal species so there is little chance of problem plants and animals remaining in the ballast tanks. This is one of the best methods for making sure that marine pests from foreign countries do not come to Australia.