Harrison, 32, from Beenleigh near Brisbane, Queensland, runs an outfit called Australian Yowie Research. He has amassed sophisticated sound-monitoring and camera equipment worth A$18,000 (HK$72,000) largely from his own pocket. ('Don't talk to my wife about it,' he says, when quizzed on where the money comes from.)

He heads a small team of part-time yowie fanatics, many of them professional people, some of them ex-military and all of them, from the look of his publicity material, excessively keen on dressing up in camouflage fatigues and spending nights camped out in the woods in search of their giant prey. 'We're a rare bunch,' he proclaims. 'We're not just hobos or backyard hicks. These people are professionals.'

That most Australians regard the idea of a species of hairy hominids living among them as ludicrous does not deter him in the slightest. He admits, with masterly understatement, that public opinion is 'divided' over whether these mythical beasts exist or not. He maintains that many people do believe in yowies, but are wary about coming forward for fear of being mocked. 'People who know the bush and who work in areas where there are yowies - police, national parks rangers, the military - all confide in us.'

Trying to pin Harrison down on his outlandish assertions is like stabbing jelly with a fork. For every sceptical question, he has a carefully prepared answer. Why, for instance, has he not managed to photograph a single yowie? 'On every occasion I've seen them it's been dark, in thick bush and they conceal themselves remarkably well. You get a glimpse, reach for your camera and then they're gone.' How does he explain the fact that not a single hair, or bone, or piece of skull, has ever been found? 'The bones dissolve very quickly in Australia's acidic soils. And I believe there is some evidence to suggest they bury their dead.' And why has he not taken plaster casts of tracks? 'They are master trackers. They walk from rock to rock and leave no footprints.'

But Harrison is not alone in his unorthodox beliefs. He was one of the keynote speakers earlier this month at Australia's first cryptozoology conference, held in a fine old colonial sandstone townhouse in the middle of Sydney. The one-day series of lectures and seminars, Myths And Monsters, brought together around 50 amateur researchers, plus a sprinkling of bona fide scientists.

The spectrum of subjects ranged from the almost credible - the existence of big black cats in areas of the Australian bush, allegedly the descendants of pumas and leopards that escaped from circuses or private collections - to the downright weird: the legend of the bunyip, a seal-like freshwater creature with a haunting cry and the tail of a lizard. Others assert the existence on the Australian mainland of the Tasmanian tiger - a kind of marsupial wolf - even though scientists insist the last creature died in Tasmania in 1936.

On the whole, the participants appear remarkably sane. This was not a gathering of gibbering lunatics. Conference organiser Ruby Lang, for instance, works as a journalist, though she doesn't use her real surname for such events in case her editors think she's lost the plot. 'There are so many stories about these creatures,' she says, 'that you can't dismiss them all. So many people claim to have had experiences or seen them - and they are quite reputable as witnesses - that you have to ask the question, what is it that they saw? They can't all be hallucinating.'

She argues that large mammals are still being found in some parts of the world. In Vietnam in the 1990s, for example, scientists discovered a giant muntjac deer, as well as a kind of forest ox. 'If something like that can remain undiscovered, why not other creatures?' she asks. 'I like to think I have an open mind, but not so open that my brain's falling out.'

With his beaky nose, unruly white hair and breathless delivery, Professor Gary Opit was one of the most engaging speakers at the conference. He claims to have seen, in the dense rainforests of Queensland in 1969, an as yet undiscovered species of large quoll, a cat-like marsupial that used to inhabit much of Australia. Such creatures are relics, he says, of a much earlier age, before the coming of humankind.

'Humans are the most efficient predators in the history of the world, especially when they teamed up with dogs,' Opit says. 'That's the reason many remnant fauna species were pushed to the brink of extinction. They were forced to be very careful about how and where they live and have developed the utmost secrecy.' Hence the continuing survival, he believes, of various ape-like creatures around the world, from the Australian yowie to the yeti of the Himalayas and the Big Foot, or sasquatch, of North America.

Opit, a respected botanist, sounds quite credible. But then he spoils this impression by displaying a drawing of his quoll, dubbed the 'North Queensland tiger' - an unlikely cigar-shaped animal with an elongated bottom.

Outside the lecture room, Peter Chapple is trying to convince anyone who will listen that Australia is home to a great number of large black cats. Rumours of escaped big cats have circulated in Australia for years, and similar sightings are regularly reported in Britain, where the tabloid press delights in speculating as to the origins of 'the Beast of Bodmin' or 'the Fenland Tiger'.

Most scientists and national park rangers dismiss such fantasies and attribute any sightings to the existence of large feral cats, which in mountain areas are believed to grow up to twice the size of a domestic tabby. Chapple, though, is adamant that many parts of Australia are home to leopard, mountain lion and lynx. He says the cats may have been introduced by Malay traders, who sold them to Aborigines long before the coming of Europeans, or were brought to Australia by American gold prospectors, who put the cats to work guarding their precious mines during the gold rush of the 1860s. There is, he says, 'lots of evidence', despite the fact that no bones or skins have been found.

At the most acceptable end of the credibility spectrum is Paul Cropper, a marketing manager, and the author of Out Of The Shadows, a book on Australian cryptozoology published in 1994. A friend of Harrison's, he admits with a smile that corroboration for the yowie-hunters' stories is 'slim'. Cropper is more interested in Australian folklore and what these mythical beasts say about society and its beliefs.

The yowie, he says, is 'the archetypal wild man of the woods' - the sort of creature that in Europe during the Middle Ages encapsulated all there was to fear about the impenetrable forests and wild lands that lay beyond the inhabited world. He says the yowie figures prominently in many Aboriginal myths. The name comes from the Yuwaalaraay language: yuwi means 'dream spirit'.

Perhaps the belief in such creatures reflects the brevity of European settlement in Australia. When the early colonists arrived, they left behind the fairies, elves, hobgoblins and other mythical spirit creatures of their home countries. They found a country rich in Aboriginal folklore, which they then adapted and embellished.

Cropper insists it would be foolish and narrow-minded to write off altogether the existence of yowies, bunyips and Tasmanian tigers. 'Either we are looking at some mass psychological weirdness among a lot of people, or there is something out there,' he says.

Lang agrees. 'Quite rightly people ask, 'Can you prove this?' Often you can't. But at the same time you can't dismiss the evidence of people who are credible and normal.'

Inside the lecture hall, Harrison holds his audience spellbound. He's recounting yet another yowie experience, this time from rural New South Wales. 'There's a bloke up at Taree [a New South Wales town] who has about six of them on his property. They just wander around during the day. Unbelievable.'

On that last point, at least, most Australians would agree with him.