The court had nullified the result of the by-election which he won after Demosisto leader Agnes Chow challenged her disqualification from the race.
Au’s appeal effectively allows him to keep his seat in Legco, at least until the court decides on his appeal.
In his appeal, Au said it was not fair that voters or himself bear the consequence of an administrative mistake made by the Hong Kong government when they were screening election hopefuls.
Au denied it was just a political move. He said the court should decide that in the event of an administrative mistake, whether that automatically triggers a by-election, or explore if there is room for the courts to grant relief to others.
The opposition lawmaker said he is not sure if the government is also planning to appeal against the High Court decision.