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Mirrielees, who played 43 games for Otago as a hooker, is leading a new elite performance programme he has developed for school pupils.
Along with that he is also teaming up with former Otago players Eben Joubert and Hale T'Pole to coach the Harbour premier rugby side for this season.
Mirrielees, who last played for Otago in 2010, said he came up with the elite performance programme as a way to help young people get fit and ready for sport.
A personal trainer, he said many young people did not know how to get to the next level in sport and that often came down to training.
He had trained for up to three hours a day when he was at school at Southland Boys' High School but had not improved because it was not the right kind of training.
Things such as nutrition, sleeping patterns and getting the right mind set were important factors for young players, and big improvements could be made.
He had worked with a couple of first XVs already and their coaches had already seen a marked improvement in the players' fitness and attitude. He was hoping to extend the scheme out to other school teams through his Let's Go Fitness company.
Joubert will also be involved in the elite programme and the duo will join T'Pole in coaching the Harbour team.
The Harbour side made it all the way to the final in 2011 but missed the top four last year.
Mirrielees said he was looking forward to the new coaching role and would still be turning out for the side, as would T'Pole.
T'Pole has played for Otago and Southland as well as Tonga.
He was called back into the Highlanders last year as training cover after spending a few years playing in Japan.