"Sydney" sailed again from Adelaide on 26 March for her nineteenth voyage, carrying general cargo and foreign aid supplies for the Khmer Republic. She met the destroyer "Duchess" off Singapore on 3 April, and the two ships arrived in Vũng Tàu on 5 April. The voyage officially ended when "Sydney" and "Duchess" arrived in Hong Kong on 8 April, and after a short period of recreational leave, the troopship returned to Australia. The twentieth voyage saw "Sydney" sail from Townsville on 13 May, with 4 RAR embarked. She met "Duchess" and "Parramatta" en route, and reached Vũng Tàu on 22 May. The ships left a day later, with 2 RAR aboard for the voyage home. "Sydney" arrived in Townsville on 1 June. In July 1971, "Sydney" sailed to Esquimalt, British Columbia, for Canada's centennial naval assembly. She then visited San Diego to collect ten new A-4G Skyhawk aircraft for the Fleet Air Arm, and delivered these to Australia in mid-August. On 20 September, "Sydney" departed on the twenty-first voyage to Vietnam. The troopship first sailed to Singapore, where she offloaded equipment for the ANZUK force and met the destroyer escort "Swan". The two ships were in Vũng Tàu for 6–7 October, during which 3 RAR was heli-lifted to "Sydney" for return to Australia, with the troopship arriving in Adelaide on 16 October. Ten days later, "Sydney" sailed from her namesake city for her twenty-second visit to Vietnam, with a cargo of Defence Aid stores. Meeting the destroyer escort "Derwent" en route, "Sydney" arrived in Vũng Tàu on 6 November, where the stores were offloaded and replaced by Australian personnel and equipment from assorted units over a two-day period. Arriving back in Sydney, the troopship left again on 24 November for her twenty-third voyage to Vietnam. "Sydney" met "Swan" off Subic Bay, then arrived in Vũng Tàu on 8 December, where 4 RAR, the 104th Battalion of the Royal Australian Artillery, and No. 9 Squadron were embarked for the return to Australia. The ship was visited by South Vietnamese President Nguyễn Văn Thiệu before departing, who thanked the efforts of the Australian military during the Vietnam War. "Sydney" arrived in Townsville on 17 December.