A Squirrel AS 350B helicopter, and a Jetranger 206B helicopter, were conducting scenic flights in the Sydney area on the afternoon of the occurrence. These flights usually passed over Middle Harbour but, because runway 25 was in use, they were cleared to track in company via the less familiar St Peters route. The Squirrel, being the faster of the two aircraft, flew in the lead. Nearing the end of the flight, both aircraft were asked to hold for ten minutes at Rose Bay. Both pilots later said that they became concerned because it was almost at the end of daylight, and neither helicopter was certificated to undertake commercial night operations. After holding for about seven minutes the tower cleared both aircraft to track to Sydney Airport via St Peters, and requested the pilots to expedite their return. Whilst tracking inbound, the Jetranger began to fall behind the lead helicopter. The pilot of the Jetranger did not advise the tower, nor the pilot of the Squirrel, that he had fallen behind. As he approached to cross runway 25, the pilot of the Jetranger observed a fixed wing aircraft on final for that runway, and slowed his helicopter because of wake turbulence concerns. The tower, on hearing the Squirrel report on the ground at Helipad H1 on the southern side of runway 25, assumed that the Jetranger was also about to land. At the same time a Sikorsky S-76A helicopter was inbound from the Harbour Bridge for Helipad H4, on the northern side of runway 25. A short time later the pilot of the Sikorsky reported that a Jetranger had passed in front of him from right to left, within a distance of about 150 ft, near Helipad H4. All three pilots subsequently reported on the difficulties of sighting other aircraft against the city lights. Although the pilot of the Squirrel was aware of the proximity of the Sikorsky, the pilot of the Jetranger was not. Nor was the Sikorsky pilot aware of the proximity of the other two helicopters. The aerodrome controller did not see the Jetranger against the background of aircraft and city lights, and was consequently unaware of the potential traffic confliction.