Marine conservationists say the interception of a Papua New Guinea fishing boat laden with four tonnes of "de-finned" sharks in Australian waters shows illegal shark fishing has become a worldwide enterprise and the Great Barrier Reef could be its next target.

On Thursday, Australian Customs authorities escorted an illegal fishing boat to Cairns after it was spotted 6.6 nautical miles east of Ashmore Reef an Australian Nature Reserve west of Darwin.

The boat was first detected by a Customs Coastwatch aircraft last Sunday and was intercepted.

A search a revealed about four tonnes of sharks, with their fins cut off.

Twenty shark jaws, 20 tuna and 20 assorted large fish were also allegedly discovered on board. Customs could not confirm whether the vessel's 15 crew members had been charged.

Marine conservationist, researcher and Fox Shark Research Foundation director Andrew Fox said a shark finning industry in Papua New Guinea had previously been unheard of and the fact the vessel was from the region showed the industry had become truly international.

He said illegal finning had become more lucrative as the shark population became more scarce and was now attractive to developing fishing nations.

''We are very concerned about the Great Barrier Reef,'' Mr Fox said.

''There have been moves in our own Great Barrier Reef to set up a shark finning industry because it is such big money.

''While these moves have so far been blocked, it is very scary to think the question was even posed.''

He said sharks were vital to maintaining the health and balance of marine eco systems and without them the impact would be devastating.

''Even considering a shark finning industry in Queensland is so short term compared to the long-term beauty of the reef and tourism industry that it feeds.''

Shark fins are farmed for sale to restaurants to make soup, considered a delicacy in Asia.

The World Wildlife Fund has calculated that 100 million sharks a year are killed to feed the industry and says this is not sustainable.

Customs Border Protection deputy commander Demetrio Veteri said illegal foreign fishing posed a number of risks to the sustainability of Australian fish stocks.

• Brisbane Times article