In the 2018 Legislative Council by-election triggered by oath-taking controversy, Au replaced Demosistō candidate Agnes Chow who was disqualified over her alleged pro-independence stance.

He participated in the students union when he studied at the Chinese University of Hong Kong (CUHK) and joined the anti-Express Rail Link protests in 2009 and 2010.

[3] Au was always at the progressive end of the spectrum in the Democratic Party, urging it to take a more radical stance to uphold the interests and core values of the Hong Kong people.

He had a frontline role in the 2014 Hong Kong protests, starting with the storming of "Civic Square", the public forecourt of the Central Government Complex on September 26, in the lead-up to the mass sit-ins.

[5] In 2014, he challenged incumbent chairwoman Emily Lau again in the leadership re-election in a four-way contest with legislator Wu Chi-wai and party treasurer Stanley Ng.

As a partner in his mother's fashion retail company, which has two shops and a booth in the Sincere Department Store, he is eligible to run in the trade-based constituency.

[9] He was later tipped as a candidate for the 2018 Legislative Council Hong Kong Island by-election for the seat left vacant after the disqualification of Nathan Law of Demosistō.

[12] Ko Chun-pong, a Superintendent of the Hong Kong Police Public Relations Branch, alleged that he felt discomfort in his right ear because Au's speaker seemed too loud.

[13] On 18 April 2020, Au was arrested again, as one of 15 Hong Kong high-profile pro-democracy figures, on suspicion of organizing, publicizing or taking part in several unauthorized assemblies between August and October 2019 in the course of the anti-extradition bill protests.

[17] Since his conviction, the Hong Kong Bar Association launched an inquiry against Vivien Chan Man-wai, the prosecutor in Au's case, after she was accused of violating professional standards.