Solomon Islands police said the incident occurred in the Tenaru area, about 12 miles to the east of capital Honiara. A military squad specialising in the disposal of explosives was deployed to the site shortly thereafter.
According to preliminary information, two men were trying to dig the shell out when the explosion occurred. The squad found tools used by the victims, as well as other ammunition they had managed to excavate.
The police warned that people should immediately report such discoveries to the authorities.
During World War II, the Solomon Islands saw fierce battles between Japanese and Allied forces, including the Battle of Tenaru in 1942. The areas of past battlefields are still strewn with dangerous unexploded ammunition in the ground.