When dawn broke, the bedraggled people clinging to ship watched in amazement as the two emus waded across the expansive shallows to a sandy cay about two kilometres away. ''The survivors realised they could follow,'' said Mr Hosty, the leader of the underwater archaeological ''dig'' that has just found the remains of the wreck. ''They all staggered, swam or floated to the sand cay, rescued by a couple of emus who were no doubt subsequently eaten.'' Mr Hosty, a curator at Darling Harbour's Australian National Maritime Museum, was project director of the 33-strong team which finally found the anchor and other remains of the Royal Charlotte last month - only the third wreck of a ship that carried convicts from Britain to Australia identified around the Australian coast. ''The two previous wrecks of convict transports discovered are off southern NSW and Tasmania,'' Mr Hosty said. But the Royal Charlotte - carrying 75 soldiers and officers, up to 25 of their wives and children, plus a crew of 20 under the command of former Royal Navy officer - is significant for another reason. It is early evidence of the little-known 19th century trade routes between colonial Sydney and Asia - and India, in particular.

After delivering its convicts to Sydney, Mr Hosty said, the Royal Charlotte was commissioned to transport British troops to their new posting on the subcontinent. ''One of the reasons we're working in the Coral Sea is to explore the trade links between Sydney, Calcutta (Kolkata) and Bombay (Mumbai),'' he said. ''The Royal Charlotte was built in India and was part of a much bigger picture … Alcohol and clothing came by ship to Sydney. Timber and coal went to India. There was a cultural exchange - of people, ideas, thoughts, goods and services. Many Anglo-Indians in particular ended up in Australia because of our more blended culture. Now we're doing the same thing.'' It is only in the past five years that marine archaeologists such as Mr Hosty and his colleagues from Australian universities, who came together on the most recent venture, have been able to concentrate on substantial digs around the Australian shore. Previously it has been prohibitively expensive. But Silentworld Foundation, a non-profit offshoot of an contemporary Australian shipping company that operates in the south Pacific and the Caribbean, has provided the funds for recent academic-led marine excavations. ''Maritime history is a significant part of our cultural past,'' Mr Hosty said, pointing out that until cheap air fares most Australians arrived by sea. ''We understand the cultural links between Australia and Ireland. Yet other longstanding links between Australia and India, for example, are less well known.''

But all this was new in 1825. When the Royal Charlotte smashed into Frederick Reef (about 200 kilometres off modern Gladstone), it was still the outer edge of the known world. The ship's only longboat was launched under the command of the first officer to raise the alarm at the new settlement at Moreton Bay. Meanwhile, the ingenious survivors built an elevated makeshift settlement - including forge and saw pit - on the sandy cay, which was submerged at high tide. One 19th-century woodcut, based on witness reports, survives. Still, when Mr Hosty's team began their dives last month, there was considerable doubt whether they would find the wreck. The 20-kilometre by seven-kilometre reef is in much the same spot after 186 years but the sand cay has moved with the ocean. Where to start? ''We tried to track the motion of the sand cay through information from the hydrographic office in Wollongong and other agencies,'' he said. ''But we were very fortunate. We started finding shipwrecked material within the first day.''