VH-RMH departed runway 25 and was instructed to maintain runway heading and 3,000 ft before being given a right turn. The aircraft was under the control of Sydney Departures. VH-JRY was being vectored to Bankstown by Sydney Approach South where one controller was undergoing familiarisation while being supervised by a rated officer. Approach was given a 4,000ft altitude restriction by Departures due to runway 25 being used for departing aircraft. As VH-JRY came into the overhead Bankstown position, the Approach controller descended the aircraft without co-ordinating with Departures. When the Departures controller realised the situation, he gave VH-RMH a further right turn for separation. Separation standards available to the controller were three nautical miles lateral or 1,000 ft vertical. Analysis of recorded radar data indicated that the closest point between the two aircraft occurred at 1422.48 hours when the vertical distance was zero and the lateral distance was 1.7nm. The controller undergoing familiarisation qualified as an Approach/Departures controller in 1978, although he had not exercised his rating on a regular basis for nearly four years. He had recently been selected for Check Control duties on Approach/Departures. It was the controller's first familiarisation shift where runway 25 was in use for departures and runways 25 or 34 for arrivals. He remembers being given the 4,000ft restriction by Departures and writing it on the strip, but he did not underline it (indicating the restriction). When co-ordination was received identifying the position of VH-JRY relative to radio station 2RN, he became momentarily confused as the radio mast referred to had undergone a name change since he had last actively been an Approach/Departures controller. This confusion occupied his mind until he suddenly realised that VH-JRY was in need of descent. The controller misidentified the VH-JRY radar return as being on the western side of Bankstown, when in fact it was on the eastern side. He then forgot to co-ordinate with departures and descended VH-JRY. The Departures controller reacted as soon as the situation was realised but not before a breakdown in separation occurred. Significant Factors 1.The approach controller undergoing familiarisation did not co-ordinate descent for VH-JRY below the previously advised altitude limitation. 2.The rated approach controller did not exercise an adequate level of supervision.