Australian chef de mission Kitty Chiller released a statement confirming the incident.
Chiller said the pair left a nightclub in Copacabana in the early hours of Wednesday morning (Rio time).
“McKeon chose to stay the night with swimming friends in the Copa district without informing team management,” the statement said.
Palmer has told officials that after his friend left he was approached by a man who forced him to withdraw $1,000 in cash from a nearby automatic teller machine.
“Palmer was found later by two businessmen in the Copa district who reported that the swimmer was disoriented and had lost his wallet and phone.”
The businessmen phoned the Australian Consulate who contacted the head of security for the Australian Olympic Team, Greg Nance.
Chiller said the pair must remain in the Village between 8:00pm and 8:00am local time and only travel to sporting events in official Rio 2016 transport.