[1] His operatic repertoire ranges from lyric to German-Italian-French cavalier and heroic baritone roles to masses, oratorios and classical Lieder, and also includes operetta and modern music.
In 2000, he was a scholarship holder of the Bayreuth Festival and in the following years participated in master classes with Bernd Weikl, Axelle Gall and Gottfried Hornik and Renate Holm.
He was a semi-finalist at the 21st International Hans Gabor Belvedere Singing Competition in 2002 and was engaged from there to the "Linzer Landestheater", where he made his operatic debut in October 2002 in the European premiere of The Voyage by Philip Glass under the conduct of Dennis Russell Davies.
Since September 2005, Unterreiner has been engaged as a soloist and permanent ensemble member at the Vienna State Opera, where he has been cast in 14 premieres and revivals.
He took on roles as Consul Sharpless, Lord of Faninal (Der Rosenkavalier), Donner (Das Rheingold), Harlequin (Ariadne auf Naxos), Grégorio (Roméo et Juliette), Meister Ortel (Die Meistersinger von Nürnberg), Schaunard (La Bohème), Antonio (Le nozze di Figaro), Brétigny (Manon) as well as Melot (Tristan and Isolde), Johann (Werther), Fleville (Andrea Chénier), Hortensius (La fille du régiment), Shchelkalov (Boris Godunov), Angelotti (Tosca), Baron Douphol (La traviata).
In October 2008 he toured Asia with the Vienna State Opera production of Le nozze di Figaro among others in Shanghai, Seoul, Taipei, Singapore and Hong Kong.
In April 2012, he made a guest appearance at Opera de Nice in Tristan und Isolde and as Papageno in The Magic Flute.
[12] During the 2020 COVID-19 pandemic and during the lockdown in Austria, he was involved in various initiatives for music, art and culture, singing as part of "Wir spielen für Österreich" diverse Konzerte und Opernvorstellungen for the ORF.
[13] Since he himself was blinded by an eye disease as a child and only after lengthy treatments was it possible to overcome his disability, Unterreiner has been regularly involved in charitable organisations and projects at home and abroad.