Once marine pests reach Australian waters they start taking hold in the location they were released. Usually there will be barriers so these pests on their own can’t spread across all Australian waters.
However these hitch hikers are able to get a new ride on the many vessels that use Australian waters. The hulls of all vessels no matter how small will be eventually and often quickly fouled with marine life. These marine plants and animals mostly belong to Australia, but the marine pests are proven hitchhikers.
There are also many places within boats and ships where they can live in pools of water or other surfaces such as anchors and chains.
Many commercial vessels constantly move between ports. There are regulations requiring these craft to look after their vessels so there is less risk of them harbouring marine pests.
Recreational craft can be transported anywhere. Most of these boat owners are not aware of marine pests and don’t know what to do to prevent them transporting pests on their boats and trailers.