The toads kill native wildlife such as snakes, lizards, crocodiles, turtles and water birds. When harassed they secrete poison carried in two sacs behind the head which can kill within minutes of being ingested.
The toads are expected to reach the park within weeks.
Environment experts are working on plans to erect a 15km (10-mile) fence at Gurig National Park to try to block the toads' advance.
The species was introduced to Australia from South America in 1935 in a misguided attempt to wipe out cane beetles, a pest that was destroying sugar crops in north-east Australia.
The toads kill native wildlife such as snakes, lizards, crocodiles, turtles and water birds. When harassed they secrete poison carried in two sacs behind the head which can kill within minutes of being ingested.
Northern Territory Parks and Wildlife Commission acting director Mike Butler said the toads began migrating from neighbouring Queensland several years ago and are spreading across the remote province at about 100 kilometres (62 miles) a year.
Wildlife officers fear the toads will disturb hundreds of traditional Aboriginal communities in the Northern Territory, killing off their traditional food sources.
Living with toxic toads
"The short-term impact is disaster," leading Australian amphibian specialist Mike Tyler told Reuters news agency.
"Large numbers of creatures are going to die and I think it's fanciful to believe everything will recover and we'll all learn to live with cane toads," he said.
However, Northern Territory wildlife officer John Woinarski said there was evidence suggesting that the toads' impact was relatively temporary.
Within five years most species - perhaps bar the quoll, or native cat - would probably have learnt either to avoid them or to eat them without being poisoned, he said.
Meanwhile, the toads will flourish in the region's abundant ponds, or billabongs, feeding off teeming insect life, Mr Tyler said.