Sheehan, 45, who now lives in Queenstown with his three young children and wife, Laura, had an 11-year professional career in Australia, initially playing rugby league for the Brisbane Broncos, Manly Sea Eagles and South Sydney Rabbitohs before switching to rugby union.
Between 2005 and 2013 he played for the Reds, the Waratahs and the Force, and made seven appearances for the Wallabies between 2006 and 2013.
During his pro career, alone, the scrum-half sustained more than 30 concussions, some of which required hospital treatment.
Speaking at a laugh-a-minute event at Arrowtown’s Lake Hayes Pavilion on Friday night, where he was joined by former All Blacks Justin Marshall, Kees Meeuws and Stephen Donald, raising money for the under-13 Basin Barbarians to attend this month’s Global Games in Taupo, Sheehan silenced the crowd when he spoke about being "diagnosed with premature CTE".
The condition is a neurodegenerative disease caused by repeated head injuries, including concussions, and leads to the breakdown of brain cells over time, resulting in dementia-like symptoms such as memory loss, confusion, impaired judgement and mood changes.
Last week, former Highlander Shane Christie, 39, died in Nelson.
He was forced into retirement at the age of 32 following a series of concussions — his death in a suspected suicide came two years after the death of his close friend Billy Guyton, who had a brain injury likely to be connected to repeated head knocks.
In March 2021, Sheehan told Australia’s The Sunday Programme he first started experiencing "dull headaches", then vision issues a couple of years later, followed by sensitivity to loud noises, then forgetfulness and confusion.
After he retired from rugby, he experienced anxiety and depression and became suicidal "a number of times", while he also became irritable, paranoid and manic.
While CTE can only be diagnosed through an autopsy after death — Sheehan has promised his brain for research — he was treated by Macquarie University’s Dr Rowena Robbs for early signs of the condition.
At Friday’s event, he said testing had revealed scarring on his brain from repetitive concussions.
"I first got diagnosed and started to see people at 41.
"I was told by a neuroscientist that I had the brain age of a 65-year-old."
Sheehan said he loved rugby, and it had given him "everything in my life — my happiness, my wife, my kids, my friends".
However, he was passionate about making the game safer for the next generations, noting a person with ADHD was twice as affected by a concussion than anyone without that condition, while females were three times more affected than males.
"We need to ... help kids, and parents need to learn that if you see a kid get a concussion, you take them off the field, because I’m a living example.
"If you’re a coach and you see a kid get a concussion, take them off the field."
"Rugby Legends Night" organiser Nicki Perry said about $21,000 was raised during Friday night’s event, which was attended by about 130 people.
