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She was a 5-year-old, sitting at a piano, feet dangling over the edge of the stool, preparing to compete for the first time in the Dunedin Performing Arts Competition Society Instrumental Festival.
This week, the 33-year-old is back - but this time, her feet are firmly on the floor as the festival's adjudicator.
''It's far less nerve-racking being on the other side of the desk.
''It's really nice to be doing it and remembering what it was like for me - how nervous I used to get, to the point of being ill.''
These days, the former Dunedin conductor is artistic director of the Horizont Musik-Kollektiv, in Berlin, and co-director of the Reuleaux Ensemble, in London.
And over the next 12 months, she will take up one of the world's most coveted fellowships for emerging conductors, at the Royal Scottish Conservatoire, in Glasgow.
The fellowship comes on the back of her recent work with some of the world's leading orchestras, and she will have opportunities to work with the BBC Scottish Symphony Orchestra, Scottish Opera, Royal Scottish National Orchestra, and Red Note Ensemble.
Dr Mathieson says competing in the Dunedin Performing Arts Competition Society Instrumental Festival, and others like it, helped steer her towards the career she now enjoys.
''I think learning whether or not you can handle that intense pressure and focus, and the discipline of it, is an important part of cluing into whether or not there is any point in trying to make this a career."
While many disagree with the competitive nature of music competitions, Dr Mathieson believes they are a positive thing.
''I think with the way the New Zealand education system is right now, it's incredibly important that there are still areas of New Zealand life in which children are taught about excellence and attainment in competition, and trying to improve at something.
She said auditioning was a big part of becoming a professional musician, and often there was only one seat up for grabs.
''The competing element of music never ends.''
Dr Mathieson is now looking forward to a short break from her music commitments after a busy year.
She will return to London next week to move into a new flat with fiance Jon Hargreaves.
Not surprisingly, he too is a musician - a conductor, composer and arranger.
The pair are trying to juggle music commitments to make time available this time next year, so they can finally tie the knot.