When Main overheard the alleged victim talking about where she and her friend would go for a drink after the dinner, he allegedly remarked: "Off to find some men are we?"
She said: "His demeanour had changed, something had snapped, there was something angry about him."
Main followed her into a different room at the club, stood over her and told other guests to "get the f--- out", it is claimed.
She said: "I knew his behaviour was getting worse, at that time Mr Main poured a pint of beer over me, I was drenched, my hair was so wet."
The alleged victim told how she had to walk to the toilets covered in beer while Main repeatedly called her an "Australian s--t".
Main, who attended the £30,000 (NZ$51,345) -a-year Oratory School in Reading, then pursued the victim into the toilets, it is claimed.
Prosecutor Paul Douglass told the court: "He followed her into the ladies' toilets after she had gone in to clean herself up due to having had a beer poured over her head.
"He pulled her hair, he pushed her, she tried to resist but he pulled up her skirt and smacked her approximately five times on her backside."
The alleged victim told the court: "He pulled my hair, slapping me on the head. He pulled my skirt up, calling me a s--t asking me if I was wearing any knickers."
Main slapped her repeatedly around the bottom and thighs before another member of the club intervened, the court heard.
As she left the club, Main continued to call her a s--t and yelled out: "You better start running because I am coming for you," it is claimed.
He is also said to have followed the victim and her friends to a cocktail bar to hurl more verbal abuse.
Main was a national rowing champion in 2003 and represented England at that year's Home International Regatta.
He retired from competitive rowing in 2006 having won the Thames Cup at the Henley Royal Regatta.
Main is a graduate of the University of Nottingham and Nottingham Law School and is currently employed by a global asset management firm, having previously worked for Coutts private bank.
Main, of Kingston-upon-Thames, denies one count of racially aggravated assault and one count of sexual assault. The trial continues.