I just had a look at the law and case law in NSW, Australia. Harry arguably committed common assault and arguably sexual harassment, though the key tests in court would be whether there is a complainant who alleges that the touching was unwelcome. The verified footage is rather damning that the touching was certainly not anticipated, and it looked unwelcome. But only the man whose bottom was touched/slapped by Harry can confirm that - though we see what we see with our own eyes, and police and prosecutors might examine a third-party complaint.
If the man who was touched/slapped on the buttock(s) were to complain, or someone were to complain on his behalf, in this jurisdiction there also exists the possibility of a charge of sexual touching:
Sexual Touching (formerly Indecent Assault): Under Section 61L of the Crimes Act 1900 (NSW), any person who commits an act of sexual touching on another person without consent (which includes buttock-slapping) is committing a criminal offence, potentially resulting in imprisonment.
https://www.criminaldefencelawyers.com.au/blog/sexual-touch-charges-in-new-south-wales/
Harry, as a self-considered 'people leader' and workplace opinion-giver/expert/paid speaker, has certainly breached all known norms of acceptable behaviour in NSW Australia. The man who was touched/slapped on his private bottom area by Harry was at work and, in a situation of power imbalance, has had his dignity impinged by a prince who in his own words is a member of the royal family carrying out work. And it was public and it was filmed, as Harry well knew, to be a spectacle for the world.
If that's not unethical, I don't know what is. Perhaps Harry, in his dim colonialist mindset, thinks that Bondi, Sydney is some kind of 'G'day, Bruce' fantasy theme park.