The pilot landed the Piper Aerostar with the landing gear in the retracted position. Investigation of the landing gear retraction and extension system, by the operator, failed to identify a problem. The aircraft sustained damage to the propellers, lower fuselage, landing gear doors and lower rudder. Repairs to these areas have been carried out and full retraction tests performed without fault. The aircraft was then returned to service. During the investigation, the pilot reported that when the landing gear was selected down, it did not extend. The pilot later reported to the company that she did not hear the landing gear warning horn, did not confirm that the landing gear was down prior to the landing and did not complete the pre-landing checklist. The company, in consultation with the Civil Aviation Safety Authority, has instituted a revised check program for its flight crews in response to this accident.