Following the death of the calf, the mother remained with it for hours and potentially days, touching it with her head and pectoral fin, swimming close beside it, and staring into its eyes.
The July 2025 interaction was captured by a Sea World Foundation vessel in the shallow waters of Palm Beach Reef, several hundred metres off the coast of southeast Queensland.
The behaviour was a rare window into the bonds between mother and calf baleen whales, said Olaf Meynecke from Griffith University's Whales and Climate Research Program who published a study on the interaction, released on Thursday.
"What is actually happening there is very hard to understand because it's in a different environment that we're not living in," he told AAP.
"But it does give us an opportunity to understand other species better."
The behaviours are consistent with those documented in socially complex mammals, such as elephants and primates, and indicate a strong maternal attachment and potentially grief over the loss of the calf.
Understanding how non-human animals responded to death provided insight into their emotional lives, social bonds, and cognitive capacities, Dr Meynecke said.
Similar attentive behaviour such as carrying or lifting carcasses, attempting to keep them afloat, or remaining nearby for prolonged periods has been widely documented in toothed whales, including dolphins and killer whales.
Most famously, a killer whale named Tahlequah, carried her calf for 17 days over a journey of more than 1500 km in 2018.
However, evidence was largely absent from research published on baleen whales, including humpbacks.
Due to a lack of blubber, humpback calves weren't born buoyant, meaning stillborn calves would generally sink, Dr Meynecke said.
"Of course, something like this probably has occurred before, but we just don't know about it," he said.
"When it does happen, it's really hidden beneath the surface of the ocean.
"Also if it's in an area that's deeper, maybe the whales cannot really attend it either."
While the behaviours have been interpreted as caregiving or attachment, grief remains difficult to demonstrate due to the lack of physiological or neurological measurements in wild animals.
Rescue boat captain Andrew Mulville from Sea World Foundation, which rescues whales from nets, said witnessing the behaviour was a surprise.
"At first sight, I thought it was just two whales resting on the surface, but their behaviour was unusual," he said.
"It was definitely not what I was expecting to see out at sea that day."
The researchers called for further study to better understand the cognitive, emotional, and evolutionary significance of post-mortem behaviour in large whales.
"I always think the world would be a better place if we would learn from whales," Dr Meynecke said.
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