A breakdown in separation occurred between two VFR aircraft, a twin Comanche cleared Station Pier for Essendon at 1,500 ft and a helicopter cleared World Trade Centre for Essendon on climb to not above 1,500 ft. Essendon Tower passed traffic to each aircraft but the helicopter pilot claimed not to have heard advice on the Comanche. The Comanche pilot never saw the helicopter whereas the helicopter pilot saw the Comanche pass about 200 ft overhead. Significant Factors The following factors were considered relevant to the development of the incident: 1. Both aircraft were cleared at the same altitude. 2. The two pilots did not take adequate action to ensure separation was maintained. 3. Air traffic control did not provide sufficient information to the two pilots to assist them to sight each other's aircraft.