A family discovered what is believed to be a 1941 Japanese grenade and spent ammunition while walking on a beach between the Broome port and Gantheaume Point last Tuesday.
Senior Sergeant Brendon Barwick, of Broome police, said the family collected the ammunition casings from tidal pools before discovering what is now suspected to be a grenade.
"It is possible that recent heavy rain and rough weather conditions unearthed the items," Senior Sergeant Barwick said.
"The couple took the casings and object home and became suspicious of it and called police."
Photographs of the objects were sent to the Police Bomb Response and Military Ordnance Unit which identified them suspect device as possibly being a WW2 Japanese hand grenade.
Police have recovered the grenade after packing it sand and are waiting for a military ordnance team to arrive in Broome on Thursday to dispose of it.