He beat 34,000 applicants to secure the position, which came with a A$150,000 salary and a luxury villa on Hamilton Island, in Queensland.
Mr Southall took up the role accompanied by his then girlfriend and spent the next six months travelling around and promoting the area.
Nearly three years later what is he up to now?
When the charity fund-raiser, from Petersfield, in Hampshire, headed out to the Whitsunday Islands he was already a seasoned traveller who had completed solo expeditions around Africa.
Mr Southall flew to Australia with his then girlfriend to take up an unusual job sampling the tourism attractions dotted around the Great Barrier Reef.
Part of his role involved updating a blog with photos, videos and diary entries documenting his experiences to help promote the area as a tourist destination worldwide.
But the job itself was not what he first imagined it would be.
“At the time I saw the advert I thought it would be like living on a desert island like Tom Hanks in Castaway,” he said.
“It became bigger the further it went on. It was after I won when I did a 10-minute slot on the Oprah Winfrey show which was broadcast in 140 countries that I thought ‘This is pretty big now’.”
The work led to long hours as Mr Southall, who is now 37, tried to visit as much of the region as he could.
He said: “I put in a lot of work, it should have been entitled ‘the busiest job in the world’.
“Everyday was a different experience: jet skiing, staying in five-star resorts, diving – and then writing about it.
“I self-imposed too much on myself because I’ve got a strong work ethic.
“There are 150,000 people working in tourism in Queensland so the pressure was quite high because of the responsibility I felt for them.”
The job was also not without its risks – not least when Mr Southall was stung by a Irukandji jellyfish, its venomous sting can be lethal.
But despite the pressures and the brush with one of Australia’s deadly creatures, overall his experience was positive.
And while many claimed the job was a PR stunt Mr Southall took the role seriously, evidently impressing his employers who he now works for as a “tourism ambassador”.
The role sees him promoting the entire state instead of just the area around the Great Barrier Reef.
He said: “It means I’m travelling more around the state of Queensland, into the outback, the rainforest and down to the Gold Coast.
“I’m a roving reporter, I produce and shoot video content and run a blog.”
He has also done a six-part series with National Geographic and was involved in an Australian children’s show called Totally Wild.
But he had not undertaken a large-scale expedition since his time in Africa and was missing the challenge of a bigger project so in May 2011 he set off on “the best expedition in the world”.
The adventurer paddled, pedalled and sailed 1,600km over four months in a “Hobie” kayak from the town of 1770 to Cooktown along the Great Barrier Reef – retracing the route Captain James Cook took more than 240 years ago.
“I wanted to show people the Great Barrier Reef is alive and well – people think it has died off,” he added.
The expedition has kept his wanderlust alive and he would like to go on further adventures around Asia.
But unlike his solo travels around Africa he is unlikely to undertake his next trip alone as later this year he will get married.
He said: “I came out here in a relationship, that finished a couple of years ago.
“I was over here by myself then 18 months ago I was at a tourism event and I met Sophee.”
The two became engaged and in November the pair will head to Hamilton Island, Mr Southall’s first home in Australia, for their wedding.
Looking back on the past three years he said: “I didn’t expect to stay out here. I’ve always travelled and had wanderlust, this is the longest time I’ve stayed in one place.
“Four years ago I was travelling around Africa in a Land Rover. Now I live in Brisbane which is a city, not a very big city but I’d never done city living before.”
He said he thought that if he had not won the competition he would have carried on splitting his time between working in the UK and travelling to hard-to-reach places.
“The job has allowed me to have adventures and keep exploring, write about it and learn new skills like filming, editing, presenting, public speaking and TV work,” he said.
“They’re great things to have done and if it wasn’t for the competition I don’t think I’d have done them.”
(By Emma Kasprzak for BBC News)