RA said Stannard and a group of players and team management had been at a fast-food outlet following the dinner for departing coach Andy Friend when the alleged attack occurred.
Two players in the squad, including the side’s regular captain Lewis Holland, and Friend detained the man shortly afterwards and held him until police arrived, RA said.
Police said the incident happened at about 3.10am outside a kebab shop in Coogee, a coastal suburb of Sydney, when he was “punched in the head in an alleged unprovoked assault”.
Stannard was taken to St Vincent’s Hospital in the city centre where he was understood to be “in a stable condition with head injuries”.
The suspect had been charged with assault occasioning actual bodily harm.
The Briton has been bailed on “strict” conditions until an appearance at Waverley Local Court on April 19.
Stannard, who competed for Australia at the 2016 Olympics, was made captain of Australia’s Commonwealth Games side earlier this month after skipper Lewis Holland was ruled out due to a hamstring injury.