The pilot established two-way communications with Brisbane Flight Service Unit whilst taxiing for take-off. After take-off he attempted unsuccessfully to pass a departure message - checks revealed an electrical failure. The pilot elected to return to Toowoomba and noticed enroute that the radio operated intermittently. In the circuit he selected wheels down and stated that he saw the "gear down and locked" indicator illuminate monentarily and heard the gear motor run. The aircraft was subsequently landed wheels up. Investigation revealed that the connector for the battery earth strap was broken, causing an intermittent electrical supply. The pilot had not checked the emergency gear system to verify the position of the gear.