They will leave a burn like mark on the skin, some hurt more than others. If stung, do not rub with hand, this will spread any tentacle around. Pour soda water on area, to wash any jelly bits away. Then get a plant called morning glory and mix with vinegar. Hold this over area for around half an hour. Any Thai will know what to do if you say "mangopune", Thai for jellyfish.

Officials on a popular resort island in southern Thailand have laid nets in the sea to prevent poisonous jellyfish from nearing the shore after two foreign tourists were fatally stung while swimming in waters off Koh Pha Ngan.

An Australian man has died from multiple stings by a toxic jellyfish. He died before arriving at a beachside clinic after receiving massive stings to his legs while swimming off the island's Hat Rin beach. Officials said he suffered terrible pain from the stings which left large welt marks on his legs.

"The long tentacles wrapped around his legs three or four times each. It would have been an excruciating death," an official said.

A day later, a Moroccan woman died from similar stings to her legs after being taken by speedboat to Ban Don Inter Hospital on the nearby island of Koh Samui.

The deaths on the island, which is known for hosting all-night rave parties, have prompted local authorities to warn tourists against swimming in the sea during the monsoon season, when the deadly jellyfish are frequently washed toward the beach by rough seas.

Hat Rin local administrator Prasob Tauychareon told us: "We have set up a sign board on the beach warning tourists to refrain from swimming where the jellyfish are rampant, and laid nets in the areas to prevent the lethal jellyfish from coming close to the beach." The victims were the first to die from jellyfish stings in several decades in Thailand, he said.

Australia's Department of Foreign Affairs and Trade issued a warning advising Australians to "exercise extreme care" about the safety of swimming in particular locations on the island.

"Due to the uncertainty about the species and toxicity of the jellyfish that caused the deaths, swimmers, snorkellers, and divers are advised to leave the water if jellyfish are spotted," DFAT said in the notice.

Thai police stationed on the island have since been handing out leaflets warning visitors against swimming, especially in the latter part of the day and early evening. But officials said many of the travellers were disregarding the warnings. "There is a general culture on the island that all is OK," one official said.

So make sure you protect yourself against these stingers by wearing a stinger suit or tight fitting clothes whenever you go for a swim.

What you always need to do when you are swimming or diving in a new area is ask the locals. They should be able to tell you if there are any jellyfish blooms or a sudden influx of these creatures locally. This can commonly happen after a storm or strong onshore wind.

Now if there are local reports of their presence and you still have to go into the water the best thing to do is prevent yourself from being stung. This is best achieved by wearing a full wetsuit or a suit called a stinger suit. This is made from Lycra, fits snugly and will stop the jelly from stinging you, but only on the non exposed parts of your body.

The way jellyfish feed is by enveloping their prey in their tentacles and paralysing them with "nematocysts". These are tiny bags full of venom that are found on the tentacles, and on contact with a fish or human skin they fire off, releasing the poison into whatever they are in contact with. If this happens to be you or a friend then you need to act quickly.

Get out of the water as soon as you can and have someone help get any tentacles off your skin. They must first of all stop any remaining nematocysts from firing off and the best thing for this is ordinary household vinegar. Pour this over the area affected and on any remaining tentacles. If there is no vinegar handy then there are other fluids you can use, the best of which is urine, which may seem bizarre but has good medical grounding due to its relative warmth and acidity.

Having been doused in whatever liquid try to take off any remaining tentacles with gloved hands so you don't get stings on your fingers.

Now, depending on what sort of jellyfish it was appropriate action needs to be taken.

Take a simple analgesic such as Ibuprofen and apply some calamine lotion on the affected area twice a day too. Finally, if you're diving remember to always look up when surfacing after a dive, as this is where most problems happen, and going up headfirst into a Portuguese Man'o'War is not the best way to enjoy Baja.

Next time you swim it would be wise to ask a local lifeguard for a jellyfish report. They tend to be seasonal and come in swarms into swimming areas. Finally, get a "stinger suit" to swim in. This is a 1mm lycra suit that is enough to prevent stinging from most breeds of jellies.