VH-KTF [Beech76] was proceeding on an instrument flight rules [IFR] training flight from Goulburn to Canberra and was being radar vectored by Canberra Approach control [APP] for a right downwind leg for runway 35. The pilot was maintaining 6,000 feet being the last assigned altitude. VH-CZP [Boeing 737] was proceeding on a regular public transport flight from Melbourne to Canberra and was being radar vectored by APP for a left base to runway 35. The pilot was also assigned 6,000 feet by Air Traffic Control [ATC]. APP was being performed by a trainee controller who had completed four weeks of a proposed twelve week training period. This controller was supervised by a suitably qualified training officer. At 1406 hours VH-KTF was given a left turn onto a heading of 320 degrees with the intention by ATC of using this turn to provide 5NM separation with VH-CZP who was to be given a left turn onto the localiser ahead of VH-KTF. As VH-CZP was approaching the centre line the training officer realised that the trainee was leaving the issuing of the turn instruction a little late and decided to instruct the trainee to turn VH-CZP immediately. At 1408 hours the trainee issued the turn instruction using the callsign CZF. The captain of VH-CZP then queried this instruction and with this further delay the ATC training officer decided to take over and issued instructions to both VH-CZP and VH-KTF which placed the two aircraft on diverging headings. During these manoeuvres the horizontal distance between the aircraft reduced to 2.5NM with no vertical differential. The weather conditions were such that both crews were in cloud throughout the occurrence and no pilot saw another aircraft. The required separation standard was either 5NM horizontally or 1,000 feet vertically. Significant Factors 1.The trainee approach controller allowed VH-CZP to get too close to the localiser before implementing a turn onto final. 2.The trainee approach controller used the wrong callsign when issuing VH-CZP with a turn instruction. 3.The training officer allowed the traffic situation to develop to a point where he was unable to guarantee separation once the trainee made an error.