Only 25 of the 200 entries into the high-profile competition, which was won by Danish architect Jørn Utzon, have been found by opera house chiefs who now hope to track down the remaining designs.
The appeal coincides with the 50th anniversary of the competition which attracted a wealth of international talent, including an entry from Australian legend Harry Seidler.
Opera House chief executive Norman Gillespie admitted the discovery that most of the designs were missing only came to light recently.
It appears most of the submissions were returned to the architects - most of whom will now be in their 80s - after Utzon was announced as the winner.
Gillespie said: 'These designs are part of Sydney Opera House's story. They are an important historic record and should not be lost.'
Any designs can be emailed to firstname.lastname@example.org