The pilot had just purchased the aircraft. He reported that when the landing gear was selected down, in the Maitland circuit, the nosegear failed to extend. All attempts to lower the landing gear using both normal and emergency procedures were unsuccessful. The aircraft subsequently diverted to Bankstown where an emergency landing was carried out with the nosegear retracted. It was later found that the nosegear door forward actuating linkage had travelled into an overcentre position, preventing the nosegear doors from opening and the nose leg from extending. A mandatory service bulletin was issued by the manufacturer in October 1989 to improve the reliability of the nosegear door mechanism and to minimise the possibility of the nosegear not extending. The service bulletin was not the subject of an Airworthiness Directive in this country. No evidence was found to indicate the landing gear had been modified.