The British women, Caroline Clarke and Joanne Walters, both 22, have not been seen since they left a Sydney backpackers' hotel on 17 April.
They arrived in Australia last year along with almost 45,000 other young Britons on working holidays, joining an army of young tourists who typically converge on a network of backpackers' hotels in the crowded inner-Sydney district of Kings Cross, before setting off on adventures through Australia's interior.
Many foreign backpackers arrive unprepared for the scale of Australia, a country the size of the United States but with only a fraction of the population, most of whom live in towns and cities hugging the coastline.
Millions of Australians, who grew up with legends of 19th-century explorers such as the German Ludwig Leichhardt, the Irishman Robert O'Hara Bourke and the Englishman William John Wills vanishing and perishing while seeking to open up the frontiers, have rarely ventured into the outback. But foreign backpackers tend to cut themselves off from the outside world for months on end the further they travel from the cities.
What made Ms Clarke and Ms Walters different was their meticulous attention to keeping in touch. They arrived in Australia separately, but teamed up in Sydney with plans to travel together through Victoria, Queensland and the Northern Territory. They worked in various short-term jobs during their travels, including a T-shirt factory in Sydney and a resort on the Queensland Gold Coast. Both women telephoned their families regularly, Ms Clarke's in Northumberland and Ms Walters' in South Wales.
Shortly before they vanished, Ms Walters told her family she planned to head for Darwin, in the Northern Territory, and then to China before returning to Britain. Ms Clarke wrote enthusiastically to a friend about plans to travel across the Nullabor Plain to Western Australia and to Ayers Rock in central Australia.
After 17 April, the phone calls and letters abruptly stopped. That was the day the two women were last sighted, when they checked out of a hotel in Kings Cross. Ms Walters wore two rings on her left hand, one buckle-shaped, the other with a stone, and an oval-shaped gold locket. Both carried their British passports, but Australian immigration records reveal neither has left the country. Both maintained ample bank accounts in Australia to finance their travels, but neither account has been used since that day.
After their families had not heard from them for two months, police in every Australian state launched a nationwide search and set up a special hotline for people to report any clues to their whereabouts. Earlier this month, police broadcast an appeal for help at the third rugby league Test match between Great Britain and Australia in Brisbane.
They were inundated with calls. A man reported the women were working as cooks at a construction site in the Queensland mining town of Mount Isa. There were other claimed sightings at places as far afield as Alice Springs and Yulara in the Northern Territory and the New South Wales snowfields.
The latest theory on which police are working is that the women caught a train from Kings Cross to a town south of Sydney, where they began to hitch-hike. From there, the trail runs cold.
The disappearances of other backpackers are equally bizarre. On Boxing Day last year, a German couple, Gabor Neugebauer, 21, and his girlfriend, Anja Habshied, 20, left another Kings Cross hotel to travel to Darwin. They failed to arrive, have not been seen since and their bank accounts have not been touched.
Another German, Simone Schmidl, 22, left Sydney last January to hitch-hike to Melbourne, but vanished on the way. Her spectacles and sleeping bag were found in bush near Wangaratta, a town in Victoria. Even more weird is the case of Naoka Onda, a 22-year-old Japanese tourist, who left her passport and belongings in Sydney five years ago when she flew to Queensland for a holiday and vanished.
Although police remain baffled by all the cases, some believe a common thread to many backpackers coming to grief is their willingness to hitch rides with strangers as a cheap way of covering Australia's vast distances.
Peter Dameon, manager of Toddy's Backpacker Hostel in Alice Springs, which has a turnover of 500 young Britons, Europeans and Americans a week, has noticed common characteristics among his clientele. 'Young travellers are very trusting and can fall prey to their ignorance. I'm constantly telling them to be careful out here, where the apparent emptiness of the outback can be deceptive. Then there are some who just choose to cut themselves off from their backgrounds deliberately. They fall in love, want to get away from their families or have over-stayed their visas, so they go underground.'