A baby whale abandoned by its mother in the waters off north Sydney has been put down, after veterinarians decided it could not survive on its own.

The humpback whale calf had been trying to suckle from boats, which it apparently mistook for his parent.

The plight of the starving two-tonne animal, dubbed Colin, has captivated Australia since last weekend.

The whale was first given a sedative, then a lethal dose of anaesthetic, which took 10 minutes.

“It’s a tragic end to a programme that dozens of people have put their hearts and souls into,” National Parks and Wildlife Service official John Dengate told Australian radio.