The joy-flight was proceeding northwards over the beach of Surfers Paradise and had been cruising at 500 feet for just over a minute when the engine power reduced to idle. After conducting his trouble checks and finding no way to restore power the pilot selected an area for a landing on the beach. The aircraft came to rest in shallow water after manoeuvring to avoid people on the beach. On inspecting the engine area the pilot found that a ball joint in a push-pull rod to the carburettor throttle valve had failed, allowing the throttle to close. Further examination by the maintenance organisation revealed that a plate to which the rod was attached was shaped incorrectly. This caused the spring loaded ball joint to wear abnormally until it fell off. The engine had been overhauled 200 hours earlier.