Three aircraft were in the landing sequence for runway 34R at Sydney. VH-NEK, on right downwind as number three, was given an ATC instruction to "sight and follow" the second aircraft, VH-DMO, which was on a left circuit. However, VH-NEK was seen to follow VH-SMH, the first aircraft in the sequence which was also on a right circuit. Separation was lost between VH-NEK and VH-DMO before the Approach Controller could contact the pilot of VH-NEK, who had already changed to Tower frequency before being instructed to do so. As a result, the aircraft came within 1 NM of each other before VH-NEK was vectored to follow VH-DMO.
Subsequent investigation indicated that VH-DMO, would have been in the "two o'clock" position from VH-NEK, at a range of about 10 NM, when the pilot of VH-NEK was instructed to follow the second aircraft. The first aircraft in the sequence, VH-SMH, would also have been at "two o'clock" from VH-NEK, but at a range of only 5 NM. VH-DMO was also nearly head on to VH-NEK at the time and would have been difficult to see. As a result, the pilot of VH-NEK mistook VH-SMH for the second aircraft and turned inside VH-DMO, resulting in a loss of separation. In hindsight, the "sight and follow" instruction may not have been the most suitable for this particular situation.
