Whale watchers off the coast of Ballina in northern New South Wales have held a front-row seat to the gruesome spectacle of a juvenile humpback whale being devoured by a pod of killer whales, also known as orcas.

Key points: Witnesses say they were watching a mother humpback whale, a calf, and a male, when a pod of orcas emerged

Witnesses say they were watching a mother humpback whale, a calf, and a male, when a pod of orcas emerged The orcas separated the calf from the adult humpbacks before attacking it

The orcas separated the calf from the adult humpbacks before attacking it It is unusual for humans to witness such attacks, according to a marine wildlife group

The attack happened just 2 kilometres off the breakwall at Ballina, as the local whale-watching charter was running its Saturday morning tour.

Skipper Dean Fuchs said seeing the killer whales in the wild was one of the most amazing things he had ever witnessed out on the ocean.

"Everyone was in awe and a bit of shock as well," Mr Fuchs said.

"It was an amazing experience and it was a little bit sad as well … but I guess it was all nature, part of the circle of life."

"After when they hung around, they were just super inquisitive, they were breaching next to the boat, spyhopping and pretty much playing with the boat."

The orcas surrounded the calf, which was travelling with its mother and a male escort. ( Supplied: Out of the Blue Adventures )

Mr Fuchs said a mother humpback whale, a calf and a male escort humpback whale were frolicking in the calm conditions when a pod of about 30 orcas emerged.

The orcas managed to separate the calf from the adult humpbacks before drowning and devouring it.

Mr Fuchs said the killer whales then surrounded the tour boat for more than two hours and seemed to be celebrating their kill by bringing a bit of blubber over to the boat.

The kill has been described as classic orca behaviour. ( Supplied: Out of the Blue Adventures )

Julie Cummings was on board the whale-watching boat when the event unfolded.

"We were watching the mother whale, her baby and a male having a great time splashing their tails and pectoral fins for quite a while, when out of the blue a pod of orcas turned up and started circling around the whales and jumping over and onto the baby humpback," she said.

"There was a lot of commotion and they eventually separated the baby from the mother and the male.

"They proceeded to basically smother the baby by jumping onto it and covering its breathe hole [blow hole] and holding it down. It was all so sad but at the same time amazing.

The orcas worked as a team to attack the humpback calf. ( Supplied: Out of the Blue Adventures )

"The whole boat was quiet with the occasional OMG.

"The last sighting of the baby was of it on its back with blood around its mouth … then we knew that the orcas had succeeded in their hunt.

"The mother and the … male tried to save the baby but there were just too many of them [the orcas].

"We had an amazing experience, something the grandkids, my daughters, sister and myself will talk about for years to come, and to think we weren't so keen on going that morning as it was a cold and miserable day."

The pod of orcas spent hours near the whale-watching boat off the coast of Ballina. ( Supplied: Out of the Blue Adventures )

A classic kill

Jules Farrell, vice-president of the Organisation for the Rescue and Research of Cetaceans in Australia, said the kill off Ballina was classic orca behaviour.

Ms Farrell said the animals followed the humpback pods, particularly on their southern migration to Antarctica when they were travelling with juveniles.

"They do travel in large groups. That's how they separate a calf from its mother and male escort. It's what they do," she said.

What was unusual about this kill, she said, was the orcas devouring the entire calf instead of their usual practice of just eating the tongue and the cheeks as a 'delicacy'.

The orcas killed the juvenile humpback whale then ate it. ( Supplied: Out of the Blue Adventures )

Right place, right time

Ms Farrell said given the vast distances travelled by the orcas and the speeds at which they travel, it was very unusual for humans to witness such an attack.

"For a whale-watching cruise to see this is really incredible."

The orcas came very close to the whale-watching boat. ( Supplied: Out of the Blue Adventures )

Skipper Dean Fuchs said the people onboard his tour boat were in awe of what they were seeing, but he wasn't sure if they appreciated its rarity.

"I was in shock when I first saw it happen. I was scrambling around trying to get my camera," he said.