While attending the school, he was invited to feature in the Barbara Cook and Friends charity gala show, organised for Barbara Cook's eightieth birthday, and was awarded The Stephen Sondheim Society Prize for Student Performer of the Year, making him the inaugural recipient.
[6] The actor featured in the 2006–2007 pantomime of Robin Hood and The Babes in the Wood at the Bournemouth Pavilion with three of his fellow Arts Educational College classmates.
[12] The actor played the role of Nephew Fred in the 2009 production of A Christmas Carol at the Birmingham Repertory Theatre.
[13] In March 2010, Au's casting as the mute in The Fantasticks was announced; the production began in the West End in May.
[13] Between September and October 2010, Au appeared in the West End production of Bells Are Ringing at the Union Theatre, London; he portrayed Carl.
[14] In December 2010, it was announced that Au would join the West End cast of Jersey Boys as Joe Pesci, from 15 March 2011.
[18] Daily Mirror's Clemmie Moodie praised Au's performance in Waterloo Road, opining that he managed to "effortlessly nail [his] delivery every time".
[7] In February 2013, Au appeared in the Let's Dance For Comic Relief competition alongside four of his Waterloo Road co-stars.
[3] The actor told Kate Forrester of the Liverpool Echo that he relished the chance to "dance, sing and act all at the [same] time".
Shortly after, he auditioned for the lead, Alan 'Icarus' Flynn, in the play Bright Phoenix at the Everyman Theatre, Liverpool.
[24] In September 2019, it was announced that Au would star as Dick in the UK premiere of the musical High Fidelity at the Turbine Theatre, running between October and December 2019.