Some 300 people were on board Monday's Boeing 777 flight, which was to set to last eight hours.
The crew decided to turn around after finding the toilets could not be flushed properly due to a technical problem, a spokeswoman for the airline told AFP on Tuesday.
She said, to her knowledge, such a problem had not occurred on an Austrian Airlines flight before. The plane has already been fixed and is back in service, the spokeswoman added.
Affected passengers were rebooked on other flights.