The 43-year-old had returned to the country where his life and promising rugby career was turned upside down in 1989 by a tackle that left him paralysed from the neck down.
Millar rebuilt his life back in Scotland and then England, and developed a strong interest for spinal injuries that led him to become a doctor in clinical neuropsychology.
Last year, 20 years on from his accident, he decided to take on 'The Mighty Push' with a clear target of raising more than 25,000 to help with research into spinal injuries as he looks forward to testing pioneering treatments being transferred from laboratories to clinics and human trials. Backed by the Scottish charity Hearts and Balls, which provides widespread support to injured rugby players, he reached that target before landing in New Zealand and while his 'Just Giving' website is now around the 29,000 mark, donations received in New Zealand are expected to take him well over 30,000.
Millar himself, however, finished his marathon-a-day on main roads, dirt tracks and old rail lines completely shattered.
"I am absolutely, bloody knackered," he told The Scotsman. "The last few days have been really tough; definitely the hardest of the whole experience. The Otago rail trail was absolutely terrible – a bad surface and hilly. On a normal bike it might be fine, but with three wheels it was just really tough.
"Mentally it has been very tough. I have done a few things now in my life, but this has certainly been the most demanding, physically and mentally, and now I look forward to a rest and then helping take forward the research."
Millar was given a send-off at the start of his trek nearly two weeks ago by the All Blacks captain Richie McCaw and his Crusaders team-mates and, after appearing across the national media, was interviewed on the pitch at half-time during the Super 14 match between the Crusaders and New South Wales Waratahs. And then the reality hit him – around 26 miles a day for ten days, back-to-back, using a special hand-cycle when only one side of your body really has the power to propel the bike along.
The former Merchiston Castle pupil faced all kinds of weather through the trip and averaged just under seven hours in the saddle per day, having set off from the sports injuries clinic in Burwood across the Canterbury Plains to Geraldine, 109 miles away, then 50 miles along the Lake Tekapo canal system and, the final stage, following the much rougher Otago rail trail back to Dunedin.