Au made his debut in Hong Kong First Division League for Tin Tin on 20 December 1988, against Happy Valley. He was 17 years old at the time and the final result of the match 0-2.
Au was the leading goal scorer of the Hong Kong team, but has since retired from international football after an exhibition match with Manchester United on the 23 July 2005.
Au can be recognized easily by his hairstyle - he sports peroxide-blond streaks in his locks. He wears the number 7 jersey for South China and is the captain of the team.
He is also remembered by his famous "cross passing during 1-on-1" in the final of 2002 Carlsberg Cup against Honduras. In the match, he received a pass from his teammate, and got a goal opportunity. However, he passed the ball back and did not shoot. Further jokes on this were developed after this incident.
Being a veteran striker, Au is valued for his international experience and his strong aerial ability. The 6' tall striker remains one of the best headers in the League.
In the season 2005-06, his good performance throughout the season could not prevent South China from finishing 7th in the league, with 2 points below the relegation line. He scored 8 league goals in the season (4th among all players, 1st among Hong Kong players). Two years after retiring, he chose to return and play for Eastern, one of his former clubs.