Swimming great Ian Thorpe Thursday revealed he has battled mental health issues since his teenage years as he opened up about his struggle with depression.

Australia's most decorated Olympian, with five gold medals at the 2000 Sydney and 2004 Athens Games, was admitted to a rehabilitation facility for the condition in 2014 after being found disoriented on a Sydney street.

The 33-year-old has said previously that he kept the problem secret from his loved ones, but is now part of Young Minds Matter, a campaign designed to raise awareness of children's mental health issues backed by Prince William's wife Kate Middleton.

"I am someone who has struggled with mental health issues since I was a teen," Thorpe wrote in a blog post for news website Huffington Post Australia.

"From the outside, many would not see my pain nor be able to relate to the sometimes-daily struggle I was facing.

"This is part of the deception of depression and also mental illness: what may appear at face value is a stark difference from the agony that lies within."

Thorpe, still hugely popular in many parts of the world, became the first person to win six gold medals at one world championships, in 2001, among 11 world titles overall -- along with 10 Commonwealth Games gold medals.

But the demands of a celebrity lifestyle and grinding training saw him quit in 2006.

He was unable to find a direction, dabbling in jewellery design and television while attempting a number of university courses before a comeback in 2012 in which he failed to qualify for the London Olympics.

In 2014, several months after treatment for depression, he received widespread praise by revealing he was gay in a move advocates said helped remove the stigma of homophobia in sport.

In the blog, Thorpe admitted that in the past he sometimes became "a hermit and tried to shut out the world" as he encouraged young people to recognise and confront any mental health issues.

"If you concede to your illness and accept its reality you fall into the trap of not only being depressed but also taking on the depressed mindset," he said.

Thorpe, who is reportedly dating swimwear model Ryan Channing, added that since recognising his problem and taking control of it, he was "tremendously happy".

"Even though I may at times struggle with depression, I've decided I don't want it to define me," he said.

"Although it may have taken me a while to get to this point and realisation in my life, I assure you it's worth it.

"I realise the wonder of the world and I approach each day with an enthusiasm that I haven't felt for what seems like an eternity."