Even if ships and boats never leave Australian waters, they can help spread marine pests.
There are many ships and commercial boats that are only used in Australia. Some of the largest ships are bulk carriers transporting ore. Smaller ships are used to trade with islands and between towns and cities. There are tugs and ferries. Many kinds of vessels are used for commercial fishing.
Most small Australian boats and yachts will never leave the country. There are hundreds of thousands of boats privately owned. Most are removed from the water after use while the others are moored in places like marinas.