The flight to Melbourne landed safely again at Los Angeles International Airport about an hour after takeoff, the company said in a statement.
Qantas said there were no safety concerns with the leak and that the captain decided to turn the Airbus A380 around in the interest of passenger comfort.
Water ran down stairs in the cabin and dripped onto passengers seated in the lower deck. Some people were moved to dryer areas of the plane.
"You couldn't keep going like that for 15 hours," passenger Ken Cross told KABC-TV.
Passengers were put up at a hotel as other flight arrangements were made.
The 11:40 p.m. flight was carrying about 400 passengers, LAX spokeswoman Amanda Parsons told The Associated Press.
Qantas says the plane's crew did all it could to help customers and that the issue with the plane was being addressed by its engineers.
The company said it was working to determine the cause of the leak.
The A380 is a massive four-engine jet with two decks of passenger seating. The planes first flew to the United States in 2007.
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