TWO huge box jellyfish found by lifeguards at Balgal Beach were carrying enough venom to kill 120 people.

The large marine stingers, each measuring 30cm in bell diameter, were found still alive after they washed up 60km north of Townsville on Wednesday.

The find follows that of a 16cm box jellyfish with 3m tentacles which washed ashore at Palm Cove in Cairns this week.

Lifeguards also closed South Mission Beach on Wednesday when torn off box jellyfish tentacles were found in stinger nets.

Surf Life Saving CSIRO marine stinger expert Dr Lisa Gershwin believed the jellyfish may have survived from last summer.

Dr Gershwin, who has carried out research on jellyfish around Cairns, said each of the Balgal stingers had the potential to kill 60 people, based on the number and length of each tentacle.

"It takes 3-5m of tentacles to kill an adult human in as little as two minutes, and as little as 1.2m of tentacles to kill a child in as little as two minutes," she said.

"Finding big ones is a really strong reminder that there are big, venomous ones out there, which are highly toxic and rapidly deadly," she said.