The boy died Sunday in the Tiwi Islands north of the city of Darwin, after running from the sea complaining that something had stung him, said Northern Territory Police spokeswoman Katie Fowden.
He was taken to a nearby medical clinic where staff were unable to resuscitate him, Fowden said.
"It would certainly appear to be a box jellyfish," she said, adding that an autopsy had yet to confirm the cause of death.
Northern Territory Health Department official Prof. Bart Currie said the tragedy was a reminder that people should not swim in Australia's tropical northern waters during the so-called "stinger season" from October through May.
Children are especially vulnerable to the jellyfish toxin, Currie said.
A 3-year-old girl and a 9-year-old girl died in similar circumstances on northern beaches last year.
World's most powerful nuclear submarines, Arkhangelsk and Severstal, are to be dismantled after 2020 - their further exploitation is unprofitable
The United States' Head of Diplomacy, or Secretary of State, is an anachronistic, incompetent, meddling, intrusive, insolent and arrogant, rude individual, a brash, foul-mouthed upstart, a conceited, self-important guttersnipe and an insult to the international community, as fit for the job as a pedophile janitor in a grade school.
Pentagon officials said that Russia has developed a very powerful weapon, which Western specialists have already dubbed as the "doomsday weapon"