|HMAS Sydney was deployed to Korea while the completion of Melbourne was delayed to incorporate and angled flight deck, mirror landing system and steam catapult. HMS Vengence was loaned to Australia as outlined in good old Wikipedia
"Loan to Australia
In June 1951, following the receipt of advice that the Australian aircraft carrier HMAS Melbourne would not be completed until at least March 1954, the Australian Defence Committee recommended that Australia request that the RN loan a carrier to the RAN. The new predicted date of completion for Melbourne was 21 months later than previous predictions, on which the establishment of two-carrier naval aviation in the RAN had been dependant. The Navy sought a four-year loan of an aircraft carrier from late 1952 to late 1956, to cover both the delays with Melbourne and the planned upgrading of HMAS Sydney once Melbourne was in service. The Australian government proposed that the loaned carrier be modified to operate both Sea Venom and Gannet aircraft, so that Melbourne and the loaned carrier could both see active service while Sydney was upgraded, but withdrew the suggestion when informed by the Admiralty that providing such capability would require the installation of a new arrestor cable system, prevent the loaned carrier from entering service until at least March 1954, and would be paid for entirely by Australia. Several smaller modifications were approved and paid for by Australia, including the installation of additional aircrew accommodation.
Vengeance was the carrier selected for the loan, and modifications for Australian service were completed in January 1952. In order to provide personnel for the loaned carrier, the RAN had to place the light cruiser HMAS Hobart into reserve. The loan was approved, with the British government choosing not to charge Australia for the loan, but stating that all operational costs would be met by the RAN, including the initial outfit of stores. In mid-1952, the liner Asturias was chartered by the RAN to transport a commissioning crew to England."
My assumption is instead of approaching the RN we approached the USN and were loaned an Essex. Such a ship would have provided so much more capability than even the modernised Melbourne that the RAN would likely have made almost any sacrifice to keep it including on selling the Majestics.
It could have been modernised, or traded back to the USN for an SCB-125 Essex granting the RAN an effective strike carrier into the 1990's. The RAN could maybe have pooled with the USN to ensure we always had a carrier available by rotating our ship with various USN reserve hulls as required by the maintenance cycle.