But that is not why small groups of people are sneaking around the streets of Darwin at night, armed with torches and plastic bags.

They are "toad busters" . . . committed to ridding the city of its newest arrivals, the cane toad, and deterring them, it is hoped, from coming back.

Seventy years after cane toads were brought to Queensland from South America, they have finally reached Darwin, spreading fear and loathing in Australia's northernmost capital. The poisonous toads, which have destroyed large quantities of native wildlife en route, are being found increasingly in swamps and waterways, and even in suburban backyards.

Now the locals are fighting back. Frog Watch, an organisation dedicated to looking after native frogs and fighting the foreign invaders, has been organising nightly "toad busts". Groups of volunteer trappers fan out through the city, carrying torches, traps and plastic bags.

Graeme Sawyer, of Frog Watch, said yesterday that it was still early enough to make a difference. "We're not going to stop every single cane toad getting into Darwin, " he said. "But what we'll do is get rid of them as they arrive, and stop that build-up of toads. . . so you'll never see thousands of toads around Darwin."

No one has yet been able to halt the toads in their relentless spread across the continent. Many weapons have been tested or suggested, including the "toad-blaster", a loudspeaker system that replicates the male mating call, luring females into traps. A local politician has recommended attacking the amphibians with golf clubs and cricket bats.

Frog Watch is not only organising military-style operations . . . which have taken on an added urgency as the monsoon season builds, swelling the waterways and flushing the toads into a breeding frenzy . . . it is also calling on the Australian armed forces to do their bit. Around Darwin, large areas of land are taken up by military bases, making them out of bounds to cane-toad vigilantes. The latter are concerned that the toxic pests will thus be able to breed, unfettered, in waterways on the bases.

Ian Morris, one of Sawyer's colleagues in Frog Watch, said the military could play an important role.

"We need as many people on the ground as we can possibly get, and if the military can work out strategies for controlling toads on their ground, that's fine with us, " he said.

Cane toads were introduced in 1935 in an effort to eradicate native cane beetles from the Queensland sugar cane crop. The first batch, 101 of the amphibians, were released south of Cairns, where they ignored the beetles but ate nearly everything else.

Since then the toads, which secrete a deadly toxin when threatened, have ravaged populations of indigenous fauna, including kookaburras, snakes, goannas (a type of sand monitor) and quolls (native marsupial cats). Their skin remains poisonous after death, and even dingoes and freshwater crocodiles have died after eating them.

From Queensland, the toads have been spreading across Australia. They have a voracious appetite and a rampant libido, allowing them to colonise an area rapidly. They can cover up to 30 miles a year, and are now nearly 2,000 miles from their starting point.

They have evolved bigger legs to move faster. The population is currently estimated at over 100 million.