Swimmers should be on particular alert in Waikiki, especially 'ewa of the Kapahulu groin, according to the city's Ocean Safety Division.

Other areas that have had box jellyfish invasions include Hanauma Bay, Pokai Bay, Makaha Surfing Beach and Waimea Bay.

Warning signs will be posted at beaches with box jellyfish sightings and lifeguard stations are equipped to deal with stings.

People who are allergic or concerned about stings should stay out of the water, according to Ocean Safety officials, and people who do get stung should flush the area with white vinegar.