Many Australian warships operated with the RN in the Mediterranean and even Atlantic in 1939-41, so there's a good chance our Australian carrier would have served there also. She might make some valuable contributions, and of course there's also the chance she could be lost or damaged. Force Z was attacked by unescorted bombers, so even a few Fulmars or Martlets might have made a difference. Prince of Wales was disabled by a single lucky hit, and Repulse was done in by a classic coordinated attack from multiple directions. A little bit of disruption could have prevented either of those and given the ships a better chance to defend themselves by fire or maneuver. On the other hand, once the Japanese found themselves being shot up by carrier figthers, they would probably make the carrier their prime target. One more point, if one of the British carriers was exempted from the treaty limits by being transferred to Australia, the RN would have another 22,000 tons or so available. Like the USN and IJN, they did not build their full allowance of carriers until the mid-1930s, at which point the additional ship would probably be built to the then-current design, Ark Royal, a considerable asset when war came. This would also require the Fleet Air Arm, which was under the RAF until 1939, to support several additional squadrons.