During May 1965, "Sydney" was undergoing a refit at Garden Island, when she was ordered to prepare the ship for her first voyage to Vietnam. The refit was hastily completed, and cargo loading started on 23 May. A blanket media ban concerning the ship was issued, and soldiers of 1st Battalion, Royal Australian Regiment (1 RAR) and the Prince of Wales's Light Horse armoured regiment, along with a logistics unit and a group of journalists were covertly transported to the ship on 26 May to foil any protest attempts. "Sydney" began her departure at 01:39; leaving Sydney Harbour with only her navigational lights lit. The troopship was in Vũng Tàu from 8 to 11 June, and returned to Fremantle on 26 June. "Sydney" was escorted by the destroyer for the entire voyage, and was joined by HMA Ships , , and for parts of the voyage. After loading troops and equipment in both Sydney and Brisbane, the troopship, accompanied by "Melbourne" and "Anzac", started her second voyage on 14 September. "Sydney" was handed off to "Duchess" and off Manus Island six days later. The three ships reached Vũng Tàu on 28 September, and departed two days later: after clearing the Market Time area, the two destroyers headed for Hong Kong, while "Sydney" proceeded to Subic Bay, then to her namesake city, where she arrived on 20 October. Despite these deployments, "Sydney" was later presented the Gloucester Cup for 1966; being the most efficient vessel in the RAN that year.