The aircraft was engaged on a charter operation from Merimbula to Moruya, Nowra and Bankstown. The aircraft operation was normal until the DEPARTURE from Nowra. After takeoff when the gear was retracting, the pilot noticed a momentary dimming of the cockpit lighting. The gear indications were that the normal retraction cycle had been completed with red light on. The flight continued to Bankstown but when the gear was selected down, nothing happened. The pilot then noticed the landing gear motor circuit breaker had tripped. All attempts to reset it were unsuccessful. The aircraft was then flown to the training area where the pilot tried unsuccessfully to lower the gear manually. The aircraft returned to the Bankstown circuit where it was verified by ground observers that the gear was still retracted. When all emergency services were in place the pilot carried out a wheels up landing on runway 29 centre. The aircraft slid for 325 metres before coming to rest on the runway centreline. Substantial damage occurred to the underside of the fuselage and both propellers were bent. Subsequent investigation revealed the landing gear dynamic breaking relay had failed in the UP position which allowed the motor to drive the retract actuator past the full UP position and jam it on the UP stop. An internal inspection of the relay found that the UP relay contact points were badly pitted and burnt from arcing.