An injured whale that died after being stranded in Tai O might have been hurt or fallen sick before entering Hong Kong waters, a marine conservationist said on Sunday.

Taison Chang, chairman of the Hong Kong Dolphin Conservation Society, believed it could be a rare Longman's beaked whale.

"Even before the whale entered Hong Kong waters, it could be sick and got lost, that's why it's found in Hong Kong waters. It's very possible that the whale had some kind of disease or got injured by other reasons," he told RTHK.

The Agriculture, Fisheries and Conservation Department had activated a response plan to rescue the whale after it was sighted in the waters near Tai O on Saturday evening.

"When we saw the animal near the evening time in Hong Kong, the animal was already very weak and did not behave normally," Chang said.

Authorities said the whale, with a wound on its body, was found to be extremely weak during sedation administered for further diagnosis, and it subsequently died in the early hours of Sunday.

Chang also said the conditions were unfavourable for the stranded cetacean to leave.

"When the residents in Tai O first saw this animal, the tide was very low. That's why when we saw a lot of photos or videos online, the animal was mostly stranded almost on the sand or on land already.

"It made the situation very difficult for the animal, even if the animal had the ability to swim," he said.