|Declassified documents and reports available through the national achives show that consideration was given to replacing HMAS Melbourne with a strike carrier operating F-4B Phantoms, S-2E Trackers and E-2 Tracers during the late 60's.
The leading contender, due to delivery times and cost, was an SCB 27 Essex conversion which was expected to provide 18 years service through to the mid 1980's.
The CVA-01 was also considered, with both local and overseas construction looked at. It was ruled out as it would not have been available to enter service until the mid 70's when it was believed the FAA would have been out of the fixed wing business for a decade.
We now know that Melbourne was upgraded to operate Skyhawks and Trackers into the early 1980's making the CVA-01 viable.
My assumption is instead of commiting additional forces to Vietnam Australia decided to boost our defence capability including ordering a CVA-01 as a strike carrier to replace Melbourne. She would have been built locally and entered service in the early 1980's. Her air group would have initially consisted of Skyhawks, Trackers and Seakings, but would have been upgraded to include Bug's, Vikings and Hawkeyes by the early 1990's.