Italy's players and coaches jump into the pool as they celebrate after winning the men's final match of the water polo event on Saturday. AFP

At least 10 people were injured when the structure collapsed on top of revellers around 2am, pinning people underneath, the Gwangju Fire Department official said.

Gwangju, about 330km south of the capital Seoul, has been hosting the championships, which feature swimming, water polo and diving, over the past fortnight. The meet finishes on Sunday.

Police investigators inspect a collapsed internal balcony at a nightclub in Gwangju, S.Korea, on Saturday. AP

South Korean news agency Yonhap said local authorities had confirmed nine foreign athletes were injured — four Americans, two New Zealanders, one Dutch, one Italian and one Brazilian.

All the athletes were water polo players except the Brazilian, they said.

Two athletes were treated at Chosun University Hospital, one was treated at an unnamed Gwangju hospital, and the remainder went to the athletes village medical centre for treatment for minor injuries, it said.

Australian, New Zealand and US water polo officials confirmed team members were present when the incident occurred.

New Zealand men's team captain Matt Small said the floor collapsed on top of people as they were dancing.

"We were just dancing and then the next minute we dropped five to six metres and everyone started rushing out of the club after that," he told New Zealand's Radio Sport.

"We ... fell on top of the heads of other people that were beneath us," he said, adding that, "Some of them were pretty dire cases".

Swimmers compete in the semi-final of the men's 50m breastsroke event in Gwangju. AFP

The women's water polo tournament wrapped up on Friday with the United States beating Spain in the final and Australia beating Hungary in the bronze medal match.

Water Polo Australia said some of its players were in the club but were unhurt.

"Water Polo Australia can confirm that members of the Australian women's water polo team were celebrating their world championship bronze medal win at an establishment in Gwangju, South Korea last night when part of the balcony collapsed," it said a statement. "All Australian players are safe and uninjured."

Christopher Ramsey, CEO of USA Water Polo, said it was "an awful tragedy."

"Players from our men's and women's teams were celebrating the women's world championship victory when the collapse occurred at a public club. Our hearts go out to the victims of the crash and their families."

Reuters