Is Yachtmaster automatically an Australian commercial qualification?

No. This catches people out expensively, usually after they have already paid for the training.

Yachtmaster is a recreational certificate awarded by a training scheme. Australia’s domestic commercial certificates for sailing vessels are Sailing Master Coastal and Sailing Master Offshore, and they are administered by the Australian Maritime Safety Authority under the national system for domestic commercial vessels. They are a different thing, issued by a different body, against different requirements.

Recreational Yachtmaster training can form a genuinely useful part of a route toward commercial work, and some of the skills overlap almost completely. But AMSA’s own requirements — documented sea service, medical fitness, marine radio certification such as SROCP or LROCP, and first aid — remain separate and must be satisfied on their own terms.

If your goal is to work on the water in Australia, work backwards from AMSA’s current requirements rather than forwards from a recreational scheme. Check them directly with AMSA before enrolling in anything, because the requirements are updated periodically.