The threat posed by Somali pirates is adding tens of millions of dollars to the cost of freight as many Australian vessels are choosing to travel round the Cape of Good Hope and avoid the Gulf of Aden where the pirates are operating.

Australian Shipowners Association executive director Teresa Hatch said the issue of naval protection had assumed greater urgency with the seizure of the giant oil tanker Sirius Star last week with US$100 million of oil aboard.

The Maritime Union of Australia has also voiced concern that pirates operating in the Strait of Malacca between Indonesia and Malaysia could hijack Australian liquefied-natural-gas tankers.

Ms Hatch said pirates traditionally focused on theft and robbery rather than hijacking. But the union warned that South-East Asian pirates could be emboldened by the Somali hijackings and Australia should organise a regional force to target the culprits.

Ms Hatch said the main concern over the recent hijacking of the massive Saudi oil tanker was not the size of the ship or the value of the cargo but its distance from the coast.

The tanker was at least 450nautical miles (830km) off the African coast and was bypassing the Suez Canal. ''This represents a major escalation in the threat.''

Pirates' ability to operate at distance from the coast is making it extremely difficult for existing naval ships in the area to prevent acts of piracy.

Once ships are seized, any military response is extremely dangerous.

The pirates are also becoming increasingly sophisticated, operating from mother ships that service fast speedboats which are used as springboards for the seizure of ships.

They are also increasingly well armed, with rocket-propelled grenades, shoulder-fired missiles and machine guns. The pirates are becoming increasingly bold and one of the large pirate mother ships is said to have fired grenades in a firefight with an Indian naval frigate this past week before being destroyed. But two speedboats launched from the mother ship fled the scene.

Ms Hatch said ultimately the piracy problem would require a solution on land to the instability in Somalia.

Ms Hatch said rounding the Cape of Good Hope could add three weeks to a voyage, but more often now shipping companies heading to and from Australia were choosing this route.