Physalia physalis, bluebottle or Portuguese Man O’ War
On the ocean-ward beach of Bribie Island, north of Brisbane in southeast Queensland. Despite dozens of them being washed up on the surf, there were children playing in the water.
They aren’t a true jellyfish, but a siphonophore, a colonial organism, and the venom is different to jellyfish venom. Vinegar makes it worse, the best treatment is hot salt water which denatures the venom