Tongan officials say divers have discovered a shipwreck believed to be a pirate vessel that local legend says sank in the 19th century with a hold full of treasure.

The Port-au-Prince, a British privateer, was attacked by local warriors in 1806 after arriving in Tonga and most of its crew were massacred on the orders of King Finau 'Ulukalala II.

Tonga's tourism ministry said the Tongans salvaged iron and cannon from the ship, before the king ordered it to be scuttled with its treasure still on board.

The vessel was thought to be lost until a local diver in the Ha'apai group of islands last month found wreckage that has features similar to the historic privateer.

Tourism ministry spokeswoman Sandra Fifita told Radio Australia she hoped the wreck could become a significant tourist attraction.

"It's bringing the pieces of our past together, and bringing a story that actually talks about the uniqueness of Tonga in how things started in the past," she told the Pacific Beat program.

Ms Fifita says if the wreck proved to be the Port-au-Prince, the treasure was likely to still be intact.

"It is believed that a considerable amount of copper, silver and gold is resting with the wreck, along with a number of silver candlesticks, incense pans, crucifixes and chalices," she said.

Ms Fifita said the wreck had copper cladding on its hull, which Britain's National Maritime Museum in Greenwich said meant it dated from 1780 to 1850, when such cladding was used to protect against shipworm and marine weeds.

Local divers are mapping the wreck for further study.

A diver at the site of a shipwreck off Tonga believed to be a treasure-laden British privateer that sank in the 19th century ( AFP: Matafonua Lodge / Darren Rice )

Resort owner Darren Rice, one of only two divers to have visited the site, said it was located on a reef just off the island of Ha'ano in an area renowned for its rough seas.

"There's very little left of the ship, it's been pounded by 4-5 metre (13-16.5 foot) swells for 200 years, so there's wreckage scattered all over the sea floor," he told AFP news agency.

Mr Rice was reluctant to reveal too much about the wreck's location, fearing an influx of treasure hunters.

"We want to make sure the area's properly mapped and everything that's found is photographed and documented," he said.

"If it's the Port-au-Prince then it's the most significant wreck in Tonga's history."

Mr Rice said conditions would be too rough for further dives until November or December and the first priority would be trying the verify that the Port-au-Prince's final resting place had been found.

"That's the most exciting thing to me, not the treasure," he said.

"Only one ship of that era has ever gone missing in Ha'apai, so if it's not the Port-au-Prince, what is it?"

The Port-au-Prince was originally built in France but was captured by the British and set sail from London in 1805 as a privateer, a ship with permission to attack and plunder the vessels and possessions of Britain's rivals Spain and France.

After almost two years at sea, during which it raided Madrid's settlements in Peru and plundered Spanish ships, it planned to hunt whales migrating through the Pacific and made its way to Tonga, where it met its end.

A teenage boy named William Mariner was part of the crew and survived the massacre, eventually becoming a favourite of the king and adopting the name Toki Ukamea, or Iron Axe.

He stayed in Tonga for about four years before travelling back to Britain on a passing ship, recounting his adventures to amateur anthropologist John Martin in "An Account of the Natives of the Tonga Islands".

The book remains one of the main sources for historians studying pre-Christian Tonga.

ABC/AFP