New South Wales Police reportedly said the attacker was "confronted by witnesses" who held him down as they called police.
Police said a 40-year-old man was taken to Manly Police Station and charged with "assault with act of indecency".
He has been released on bail and is due to appear in court on January 18.
Harrap has been a partner at Freehills Parent Attorneys since July 2013, after joining from Herbert Smith Freehills, where he had worked for seven years.
According to his current employer's website, he has "particular experience and interest in assisting his clients to defend or attack patents in patent oppositions before the Australian Patent Office".
"He passionately pursues his clients' interests, ensuring the highest quality intellectual property rights are obtained," the website says.
Harrap previously worked for a firm in New Zealand and is registered as a patent attorney there, as well as in Australia.
He does not have any previous convictions, according to reports.
Daily Mail Australia has contacted Harrap's lawyer for comment.