On arrival in the circuit area at Streaky Bay the pilot selected the gear down and obtained an instrument indication that the gear was locked down. While the aircraft was on final approach it encountered turbulence and the gear warning horn sounded, the gear unlocked indicator light illuminated and the nosegear down light extinguished. The pilot initiated a go-around and after unsuccessful attempts to obtain a gear locked down indication, diverted the aircraft to Parafield. On landing at Parafield the nosegear collapsed. Inspection of the nosegear revealed that the bolt which secures the nosegear retraction linkage to the drag brace had failed in the thread area due to fatigue. It is evident that the bolt failed and partially withdrew in the circuit area at Streaky Bay, giving rise to the unsafe gear condition.