TWO monster sized box jellyfish found by lifeguards at a North Queensland beach would have been capable of killing 120 people, according to an expert.

The large marine stingers, each measuring 30cm in bell diameter, were discovered alive after they washed up on the shore of Balgal Beach, 60km north of Townsville on Wednesday.

Surf Life Saving Queensland Townsville region lifeguard supervisor Russell Blanchard said the lethal jellyfish were the largest caught this season.

"I've seen them that big before, but we generally don't get them that big," he said.

"I wouldn't say it's a record, but it's about as big as they can get."

MORE NEWS

Driver's $110 fine for 'three-second' stop University student has fortune frozen by Federal Police

Special feature: Brisbane's 1974 long weekend from hell

Journalist Tracey Spicer: I'm a vain fool

Any jellyfish found by lifeguards during their routine drags in the water are usually donated to James Cook University for research purposes.

The lifeguards on Wednesday, however, did not have any buckets large enough to carry the jellyfish away from the beach, so the venomous sea creatures were buried in the sand and left to quickly die and decompose, to protect beachgoers.

PHOTOS: CREATURES OF THE DEEP

CSIRO marine stinger expert Dr Lisa Gershwin believed the box jellyfish may have survived from last summer, accounting for their sheer girth.

"We do sometimes see that size this early in the season, but they're probably last season's animals,'' she said.

News_Image_File: When box jellyfish reach this size they are highly venomous.

"When they reach this size, though, they're very large and highly venomous."

Dr Gershwin, who has carried out research on jellyfish in North Queensland, said each of the marine stingers had the potential to kill 60 people, based on the number and length of each tentacle.

She said each jellyfish could have as many as 60 tentacles.

"It takes 3-5m 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.

She said the animal's discovery was a reminder for beachgoers to swim inside stinger enclosures, and on beaches patrolled by lifeguards.

"Whenever we see even young box jellies, we need to take on board the fact they hang together in groups of mixed sizes,'' 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.

" I think the name of the game is protection."