Park authorities have set up a partial exclusion zone to protect unwitting boaters and whales, which can grow up to 16 metres long
Sydney: A trio of humpback whales -- usually bound for Antarctica this time of year -- took a wrong turn, heading up a crocodile-infested north Australian river with scientists hoping Saturday they will all return to the sea safely.
Two of the creatures, first seen in the East Alligator River in Kakadu National Park earlier this week, are already believed to have gone back to the sea, Parks Australia said.
But authorities were still monitoring at least one whale, spotted about 20 kilometres (12.4 miles) up the river on Saturday, Kakadu National Park manager of country and culture section Feach Moyle told AFP.
"The most likely explanation is that they were heading back down south and took a wrong turn," Moyle said.
"It's a bit difficult to tell if there's one or two there," he added, with the river's muddy colour making identification hard.
Although whales are sometimes seen off the Northern Territory coast, it is believed to be the first time one has been spotted in the tropical rivers of the huge world heritage-listed nature reserve, he said.
Park authorities have set up a partial exclusion zone to protect unwitting boaters and any remaining whales, which can grow up to 16 metres long and weigh 30 tonnes.
"The last thing we want is a collision between a boat and whale in waters where crocodiles are prevalent and visibility underwater is zero," Parks Australia said.
There were also concerns about boats forcing the creatures further up the river.
Although in prime croc country, the saltwater reptiles did not pose a major threat to humpbacks due to their size, Moyle said.
"Even a big croc is not going to tangle with them."
The bigger issue was if a whale became stranded, with a dangerous rescue in remote country making it hard to reach before the animal became "croc bait", he said.
"What we want to happen is, we get up in the helicopter tomorrow and then we see them waving goodbye."