CANBERRA, Australia (AP) — A humpback whale has found its way back to sea weeks after getting lost in a murky, crocodile-infested river in northern Australia, while an estimated 270 pilot whales became stranded in the country’s south.

There have been no previous recorded sightings of whales in remote East Alligator River in the Northern Territory’s World Heritage-listed Kakadu National Park, and no one can explain why at least three of the blue water mammals ventured so deep inland in a river with little visibility.

In a more common phenomenon, about 270 pilot whales were reported stranded Monday on sandbars off Australia’s southern island of Tasmania state.

In northern Australia, the last of the humpback whale trio managed to navigate its way through shallow channels at the broad river mouth and back into Van Diemen Gulf over the weekend, Kakadu National Park manager Feach Moyle said.

“It made its way out on the high tides and we’re pleased it appeared to be in good condition and not suffering any ill effects,” Moyle said.

Humpbacks follow the western Australian coast in their annual migration from the tropics to Antarctica.

A helicopter crew spotted the whale on Sunday 6 miles outside the river mouth, Northern Territory government whale and dolphin scientist Carol Palmer said.

“He or she looked really relaxed, was cleaning all the mud off itself actually, and we were just so relieved that this whale had actually made itself out of the East Alligator River,” Palmer said.

A group of recreational sailors first spotted the lost trio on Sept. 2 more than 12 miles from the river’s mouth.

Two of the whales had disappeared before wildlife authorities arrived a week later.

“Hopefully, they’re on their way or he or she is on their way to Antarctica, that it’s calling out for its friends and it catches up with them, because now is the time when all our humpback whales in the Northern Territory waters actually start to head to Antarctica,” Palmer said. “And knowing that these guys haven’t fed for months and months and months, so they’d be pretty hungry and they do the final 3,000-mile trip to Antarctica. So we’re hoping all that works out, for sure.”

It was a mystery why a whale would stray so far up a shallow river full of crocodiles. “It could have been chased up by some big sharks, or maybe it was just a wrong turn,” Palmer said.

Marine ecologist Jason Fowler was among the sailors aboard a catamaran who first saw the whales in a relatively deep river bend. By the time government scientists responded, the remaining whale had moved downstream to a stretch of the river only 13 feet deep.

“It was coming up with mud all over it,” Fowler said.

Despite the river’s name, there are no alligators in Australia. It was named after the river’s many crocodiles by European explorers who apparently couldn’t tell the difference.

Authorities thought the whale was too big to be attacked by crocodiles, unless it became weak or sick.

In Tasmania, authorities plan to launch a mission early Tuesday to rescue the pilot whales stranded on two sandbars near the west coast town of Strahan.

About 25 of the whales have apparently already died after the large pod was reported stuck Monday morning.

Tasmania Parks and Wildlife Service manager Nic Deka said marine specialists had to complete a thorough assessment before rescue efforts could start.

“When we start making an effort (on Tuesday) morning it will be with an outward-going tide,” he told reporters.

“That will be in our favor. We’ll be aiming to make the most of that window,” he added.

Wildlife scientist Vanessa Pirotta said: “To see a stranding like this is not unusual. It just doesn’t happen every day.”