Nine of the injured are foreign athletes comprising four from the US, two from New Zealand, one from the Netherlands, an Italian and a Brazilian, although none of them received any serious injuries in the incident in Gwangju city, fire officials said.
While the two killed were identified as South Koreans, most of the victims received minor injuries, except for 17 who suffered major ones,
"This is an awful tragedy... Our hearts go out to the victims of the crash and their families," US Water Polo head, Christopher Ramsey said.
The internal balcony, situated at a height of 2.5 meters (8 feet) from the main dance floor, collapsed at around 2.30 a.m. It was occupied by around 100 people at the time, Efe news reported.
Six of the injured foreign athletes were women, and all except the Brazilian are water polo players. Among the other non-athlete foreigners in the injured list are two Uzbek men.
Primary investigations suggest that the collapse occurred due to the structure bearing too much weight.
New Zealand men's water polo captain Matt Small said, "We were just dancing and then the next minute we dropped five or six metres and everyone started rushing out of the club after that."
It was "business as usual and then it literally collapsed beneath our feet", he added.
"All Australian players are safe and escaped without injury," the BBC quoted a statement from Water Polo Australia as saying.
Gwangju, located some 330 km from Seoul, is currently hosting the FINA World Championships 2019, which kicked off on July 12 and are set to conclude on Sunday.
"FINA deeply regrets the situation and sends its best wishes to any victims of this accident," a statement from the world swimming body said.
Authorities said the club had been renovated recently without official authorization and investigation is underway.
( With inputs from IANS )