Police say they have successfully disposed of a naval mine from World War II found last month in waters east of Hong Kong Island.

A diver discovered the mine 1.4 kilometres off Cape D'Aguilar near Shek O and alerted the authorities. Officers said it was the first time a complete naval mine has been found in the territory.

Six divers, two of them from the bomb disposal squad, on Friday detonated the 500 pounds of TNT the mine contained via a controlled explosion.

Suryanto Chin-chiu, an acting senior bomb disposal officer, said several shock bombs were used in advance to drive away fish, and the police had minimised the damage caused to the environment by their "military level" operation.

"When we were handling the mine, our priorities were the safety of the public and sea users. For fishes, we used several shock bombs to drive them away. Could we drive all marine organisms away? We can say a hundred percent we can't," he told reporters.

"But we can only do our best when it comes to public safety, our officers' safety and the marine environment... We already minimised the damage to the environment."

The officer said an area of seabed around 150-200 metres in diameter around the mine was damaged by the detonation.

The Cape D'Aguilar area had been off-limits for visitors, boats and planes during Friday's operation.