Earlier this week, Southall was getting off a Jet Ski in the ocean when he felt "a small bee-like sting" on his arm. When he later noticed a tingling in his hands and feet, island staff took Southall immediately to the doctor.
Progressive symptoms of fever, headache, lower back pain, chest tightness and high blood pressure led the doctor to diagnose that Southall had been stung by an Irukandji jellyfish. He was treated with antibiotics and slept off the venom's effects overnight.
"I thought I'd done particularly well at avoiding any contact with any of the dangerous critters that consider this part of the world their home," Southall wrote. "This was not what I'd wanted at all and had caught me little off guard to say the least -- I'm supposed to be relaxing in my last few days on Hamilton Island."
Australia is well-known for its myriad deadly creatures, but the peanut-sized Irukandji remains rather mysterious. It is virtually impossible to see and is tiny enough to pass through nets meant to keep jellyfish away from popular swimming spots.
Its sting can lead to symptoms including shooting pains in the muscles and chest, vomiting, restlessness and anxiety. Some symptoms can last for more than a week, and the syndrome can occasionally lead to a rapid rise in blood pressure and heart failure.
In 2002, two tourists were killed in separate incidents after being stung by the tiny creatures off northeast Australia -- the first recorded Irukandji fatalities.
"My slight knock was enough to tell me that it's not something to be messed around with," Southall wrote of the jellyfish. "I really should have been wearing a full stinger suit, as it recommended at all beaches here at this time of year."
He said he was feeling well and was enjoying his last week on the job.
The 34-year-old former charity worker bested nearly 35,000 applicants from around the world for the dream assignment that started in July to swim, explore and relax on Hamilton Island in the Great Barrier Reef while writing a blog to promote the area and earning a salary of 150,000 Australian dollars ($120,000).