He has motor neurone disease and has lost the use of his arms and is gradually being crippled.
Harley (63) played 44 games at openside flanker for Otago between 1970 and 1976.
He spoke to the Otago Daily Times when he was back at Carisbrook on Friday night to watch the Super 14 match between the Highlanders and the Blues.
"I'm a bit reluctant to talk about my health problems," Harley said.
"Lots of other people have illness and there are others who are worse off than me."
He mentioned the late Ross Mee, who became a tetraplegic after injuring his neck in a fall, and Paul Sapsford, who died in a boating accident late last year.
Harley was reluctant to give advice to other people with severe physical disabilities, but he explained how he dealt with his illness.
"I just try to keep doing as much as I can and stay positive," he said.
"I try go keep my mind active even if my body doesn't work."
The disease has crept up slowly on Harley, who felt the first symptoms five years ago.
"I have had two neck operations caused by old rugby injuries and my work at the Clyde dam site.
"The symptoms were similar to what I suffered then, " he said.
"That is what the doctors thought.
"They contemplated another operation but couldn't see anything to fix."
It was during a visit to Mexico two years ago that his health deteriorated.
He looked for answers from his doctors when he returned to New Zealand.
In March 2008, doctors diagnosed Harley as having motor neurone disease.
"I had my suspicions but it still wasn't great hearing about it," he said.
"I just had to stay focused.
"My right arm was the first to go and then my left arm played up.
"But it seems to be moving reasonably slowly and I've got to be thankful for that.
"It doesn't affect me mentally and the brain functions OK.
"It is just my body that doesn't function properly."
The illness has brought significant life changes to Harley.
He was forced to give up farming at Cambrian, near St Bathans, because of arthritis in his ankles five years ago, and he shifted to Wanaka to run the Wanaka Trading Post.
He had to give that up in August last year.
The illness to Harley has put an extra burden on his wife, Leanne.
"I'm still a bit wobbly on my feet but I've got a wheelchair and it just takes me 10 minutes to get into town from home, bump into friends for a yarn and have a cup of coffee," Harley said.
"It [wheelchair] makes me a bit independent.
"But I still depend on my wife for everything."
Harley had not bothered about Super 14 rugby until two years ago.
"I'm stuck inside for a large part of the day and the rugby gives me something to enjoy on television," he said.
Harley watched Otago play Auckland in the Air New Zealand Cup game at Carisbrook last year, and the Blues game will be his only Super 14 game at the ground this year.
Friday night brought back memories of the happy times when Harley was a key member of the great 1975 Otago team.
It won 17 if its 19 games and was named the best team in the country by Rugby News.
Otago beat Scotland 19-15, thumped Auckland 22-4 and was unbeaten on its North Island tour.
"We had a great bunch of guys in the team and we got on well," he said.
Harley kept playing club rugby for Becks until the age of 38.
"I always carried my boots when I watched our team play," he said.
"I usually ended up playing half a game."
After finishing his career, Harley coached the local school team for a few years and followed the fortunes of son Richard, who was a member of the Otago Boys first XV as an openside flanker immediately after Richie McCaw left school.
He also played a few games for Otago Country.
Harley spent two terms on the Otago Regional Council and is still a member of the Community Trust of Otago.
Don Harley fact file
Age: 63
Occupation: Retired farmer
Wife: Leanne
Home town: Wanaka
Sport: Rugby
Club: Becks
Position: Openside flanker
Record: 44 games for Otago between 1970 and 1976
Other positions: Otago regional councillor (6 years), Community Trust of Otago trustee
