The 1- to 2-month-old calf was first sighted Sunday in waters off north Sydney, and on Monday tried to suckle from a yacht, which it would not leave. Rescuers towed the yacht out to sea, and the calf finally detached from the boat, but the creature returned to an inlet near Sydney Tuesday morning, New South Wales National Parks and Wildlife Service spokesman John Dengate said.
The calf can't survive without mother's milk for long, Dengate told Australia's Fairfax Radio Network.
"It's probably a question of days rather than weeks," he said. "It's a very grim prognosis."
Artificial feeding would be impossible, so the calf's only chance is to find another female whale to accept it, he said.
"If a lactating female with a calf goes past and this calf approaches that animal it may accept it, but ... it's a very slim chance," Dengate said.