An Australian Government defence think tank says there is a credible risk of a terrorist attack on an Australian port or ship.

A study of maritime security by the Australian Security Policy Institute calls on state governments to create special police units to protect major ports.

It also calls for the Australian Government to spent $100 million over three years to modernise maritime security.

The institute says the main terrorist threat to Australian ships and ports is posed by Al Qaeda and its South-East Asia associate, Jemaah Islamiah.

The study says Australia should adopt United States standards for the security of shipping containers.

It points to the special vulnerability of 56 oil and gas rigs in the Timor Sea, the North-West Shelf and Bass Strait.