Cane toads were introduced to Australia from South America in 1932 in an attempt to combat a plague of greyback beetles in the sugar cane plantations of Queensland. The experiment was a disaster - the toads had little effect on the beetle but did manage to breed prolifically, eating almost anything in sight and killing predators with a poison from their neck glands. In the past 70 years they have spread south into New South Wales, and now west across Queensland's vast, dry interior, to the Northern Territory - poised on Kakadu's rugged south-eastern border, ready to invade one of the country's most pristine wilderness areas.

It has already reduced populations of native animals such as goanna lizards, snakes, quolls (a cat-sized, spotted marsupial) and some birds - particularly the jabiru, which eats frogs and toads.

Biologists are particularly worried about species which cannot be found anywhere else on Earth - such as the Kakadu dunnart and the sand stone antechinus, both carnivorous, mouse-like marsupials. The cane toads present a double threat. The smaller ones are eaten by marsupials and snakes, which are then poisoned by the toads' toxin. The larger toads become predators themselves. 'I've seen one which was so big it filled the bottom of a bucket,' a scientist said. 'It was in a zoo and they were feeding it live mice. It was pretty impressive.' Miles says: 'Probably the group of animals that are most affected are the insects because cane toads clear out all the insects at ground level. Insects are the most important part of the food chain.'

The cane toad has been demonised in Australia, while at the same time acquiring something of a cult status. In 1990 prime minister-to-be Bob Hawke made it an election promise to provide enough funds to prevent cane toads ever reaching Kakadu. Deliberately driving over them is an unofficial sport in Queensland, while there are reports of some people using them for impromptu football matches or as practice for their golf swings. There are even computer games in which squashing cartoon cane toads is the main aim.

Outback pubs hold cane toad races.The toads' skin is also dried and smoked for its hallucinogenic properties. There is a thriving trade in stuffing and selling them as ornaments dressed in dolls' clothes.

Such indignities are not endorsed by Professor Gordon Grigg, a cane toad researcher from Queensland University. 'All animals deserve to be treated humanely. It's not the toads' fault that they have become a pest species,' he says.

Grigg heads a team of biologists who are hoping to measure the effect the creatures have on Australia's native wildlife, particularly frogs. They have invented a solar-powered, computerised listening device which can recognise the calls of native frogs. The devices are fitted at the top of hollow metal poles, about six metres tall, on the southern boundary of Kakadu. Once cane toads arrive on the scene, the scientists will be able to gauge whether frog populations are in decline by monitoring their calls. The poles were erected four years ago; Grigg says he now has enough data to begin preliminary analysis, and hopes to release a report early next year.

Even if cane toads are shown not to have a negative effect on native frogs, many people feel that they are ugly, potentially dangerous and have no right to be in the Australian bush. There is enough poison in a toad's glands to kill a small dog. If handled by humans, the venom can cause a painful burning sensation. But the pace at which cane toads have advanced across Australia so far suggests that they will inevitably colonise Kakadu. As yet there is no way of repelling the invasion. All that the rangers of Kakadu can do is to train wildlife guides, Aborigines, resort staff and other locals how to identify and destroy cane toads. 'It's not a long-term solution, but it may help us buy time,' Miles says. Or, as another conservationist put it, 'Our unofficial policy is that we take a golf club to them whenever we can.'

Others are putting their hope in bio-engineering and predict that within the next few years scientists will be able to find a way of killing off the toads with some sort of virus - the so-called 'silver bullet' situation. The federal government has already spent around A$11 million (HK$45 million) on research, without success. It has recently announced a fresh injection of A$2.1 million into a two-year project.

Scientists believe that in its natural habitat, in countries like Venezuela and Brazil, cane toad numbers are kept low by a naturally occurring virus. The plan now is to identify the virus and explore ways of introducing it safely to Australia. Lyn Hinds, a pest control biologist, says: 'The best time to attack them is as they metamorphose from tadpoles to adults. We want to look at the genes which cause a tadpole to metamorphose and see if we can disrupt the process.'

In Kakadu, the wet season is well under way. Torrential rain drums in the background as Miles talks over the telephone. 'This is not just a problem for Kakadu,' he says. 'It affects the whole of northern Australia.

'Cane toads are going to hop all the way to the Kimberley (in Western Australia)' - a living, leaping, croaking testament to the folly of trying to control one species of animal with another.