People renting boats to try to get a glimpse of a whale in waters off Sai Kung are being urged to maintain a distance of at least 100 metres.

The baleen whale, first spotted in the area around the middle of this month, is believed to have recently suffered propeller wounds to its back.

The chairman of the Hong Kong Dolphin Conservation Society, Taison Chang, said on Thursday that only one ship should be within 500 metres of the whale at any given time.

Once a vessel gets within 100 metres, it should turn its engine off, he added.

"We're mostly worried about people who are renting speedboats to whale watch because these boats move really fast and speedboat drivers tend to circle a lot more. This, along with the noise produced by the boats, could cause distress and confusion for the whale, which could lead to more collisions, and that's a huge concern," he said.

Chang also said the whale has been resting in the area for longer than expected and it could be lost.

He said he hopes it leaves soon for safety reasons, as water traffic in the waters is quite heavy. In the meantime, he urged the authorities to set up a temporary protection zone.

The Agriculture, Fisheries and Conservation Department, meanwhile, urged people to leave the whale in peace, warning that any irresponsible whale-watching behaviour could lead to a fine.

"The department has stepped up patrols of the waters where the whale has appeared and at places including the Sai Kung Public Pier...Upon sighting of whales, the public is reminded that they should keep a distance, slow down their vessels and be aware of their personal safety to avoid accidents," a spokesperson for the department said.