Australian environmentalists trying to slow the inexorable spread of the toxic cane toad have found a "monster" male during a night-time raid on a pond near Darwin.

The toad, measuring 20.5 cm (8.1in) and weighing nearly 1kg (2lb), was one of 39 picked in a "breeding frenzy" and is thought to be the largest ever found in the tropical Northern Territory.

“It’s huge, to put it mildly,” said Graeme Sawyer of the nature group Frogwatch. “The biggest toads are usually females but this one was a rampant male ... I would hate to meet his big sister.”

The first cane toads were introduced into Australia in 1935, when 102 toads were imported from Hawaii in a failed attempt to control sugar cane beetles in the plantations of the Queensland coast.

The toads breed twice a year, laying up to 36,000 eggs in a clutch, and they have spread steadily westwards across the rest of the country. Seventy years after their introduction, there are an estimated 200 million cane toads in Australia.

They have also proved fatal to Australia's delicate ecosystems, killing millions of native animals from snakes to the small crocodiles which eat them.

As part of its 'Toad Buster' project, Frogwatch conducts regular raids on local water holes, blinding the toads with bright lights then scooping them up by the dozen.

“We kill them with carbon dioxide gas, stockpile them in a big freezer and then put them through a liquid fertiliser process," Mr Sawyer said. “It turns out to be sensational fertilizer.”

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