"Sydney" sailed to Vietnam for the third time on 24 April 1966, with units from both 5 RAR and 6 RAR aboard. She met her escorts, "Vampire", , and "Melbourne" en route: the two smaller ships accompanied "Sydney" into Vũng Tàu from 4 to 6 May, while the aircraft carrier "Melbourne" left the group as soon as they reached the Vietnam operational area. "Sydney" and her escorts arrived in Hong Kong on 9 May, with the troopship returning to Sydney on her own on 18 May. On 25 May, "Sydney" departed on her fourth voyage to Vietnam, with the remaining units of 5 RAR and 6 RAR aboard, plus equipment and personnel of No. 9 Squadron RAAF. Carrying the remaining units of 5 RAR and 6 RAR, plus equipment and personnel of No. 9 Squadron RAAF and escorted by "Yarra", "Melbourne", "Derwent", and "Vendetta" for varying sections of the voyage, "Sydney" arrived in Vũng Tàu on 6 June. The voyage officially ended with the arrival of "Sydney" and "Vendetta" in Hong Kong on 11 June. In November, "Sydney" provided assistance to the submarine , which ran aground on Frederick Reef.