[2] Notorious for its sting, C. fleckeri has tentacles up to 3 m (10 ft) long covered with millions of cnidocytes which, on contact, release microscopic darts delivering an extremely powerful venom.

Being stung commonly results in excruciating pain, and if the sting area is significant, an untreated victim may die in two to five minutes.

[5][6] On January 20, 1955, when a 5-year-old boy died after being stung in shallow water at Cardwell, North Queensland, Flecker found three types of jellyfish.

[7] Chironex fleckeri is the largest of the cubozoans (collectively called box jellyfish), many of which may carry similarly toxic venom.

[4] The pale blue bell has faint markings; viewed from certain angles, it bears a somewhat eerie resemblance to a human head or skull.

[9][11] The medusa is pelagic and has been documented from coastal waters of Australia and New Guinea north to the Philippines, Malaysia, Singapore and Vietnam.

The sting can produce excruciating pain accompanied by an intense burning sensation, like being branded with a red hot iron.

[18] Among 225 analyzed C. fleckeri stings in Australia's Top End from 1991 to 2004, only 8% required hospital admission, 5% received antivenom and there was a single fatality (a 3-year-old child).

[4] Most deaths in recent decades have been children, as their smaller body mass puts them at a higher risk of fatal envenomation.

[15] Chironex fleckeri and other jellyfish, including the Irukandji (Carukia barnesi), are abundant in the waters of northern Australia during the warmer months of the year.

[23] Until 2005, treatment involved using pressure immobilisation bandages, with the aim of preventing distribution of the venom through the lymph and blood circulatory systems.

This treatment is no longer recommended by health authorities,[24] due to research which showed that using bandages to achieve tissue compression provoked nematocyst discharge.

[27] However, this study has been criticized on several methodological grounds, including that the experiment was done using a model membrane that is much different from (and more simple than) human skin.

[31] A fully grown and sexually mature Chironex fleckeri medusa will begin trying to find a mate in the spring season.