Two divers scouring the ocean floor for old bottles and glassware made an unexpected discovery at the bottom of Sydney Harbour on Wednesday: an undetonated bomb from World War II.

The find prompted police to block off an area near Jones Bay at Pyrmont as military personnel seized the device.

An undetonated bomb found in Jones Bay, Pyrmont, on Wednesday. Credit:NSW Police

Dive instructor Tony Strazzari said he'd plunged into the water for a routine dive with friend Paul Szerenga when he detected the bomb's cylindrical shape and dart-like end nestled on the floor.