The crew of VH-KDJ taxied the aircraft for departure from runway 18. Calls were made on both the area and Mandatory Traffic Advisory Frequency (MTAF). From this it was ascertained there were two other aircraft operating in the area. Initially both were to use runway 18, but one of these advised he would land on 36 instead. This did not cause any problems for the crew of VH-KDJ and they were still able to use runway 18. Engine power was applied for take off and as the roll started they transmitted a call that they were rolling. The pilot of the aircraft using runway 36 called them and said there was another aircraft, of which they were not aware, taking off in the opposite direction. The take off in VH-KDJ was rejected at low speed and the two aircraft did not get close. The conflicting aircraft on runway 36 was a Jabiru which was being flown on circuits. It was subsequently learned that the radio on the Jabiru was defective. A carrier wave could be heard when the pilot tried to transmit a message but the pilot's voice could not be heard. Significant Factor The following factor was considered relevant to the development of the accident: 1. The VHF radio on the Jabiru was unserviceable.