I think it goes back to the early 1800's when anyone could claim to be an engineer. Buildings and railroad bridges etc. collapsed and it was a problem back then, nevermind the Sydney Opal tower fiasco today. Australia is likely trying to copy other parts of the world with the Act.
The disadvantage is high membership dues ($415/year) in order to support the fat and overhead of a bloated association that largely does nothing for the profession beyond registration and licensing. Here the engineering association keeps trying to include software engineering, "stamped" so you are responsible for software that can cause injury or loss, even though software correctness is not provable.
As I've said many times, there still is a problem in the profession in that anyone can call themselves "engineering manager" or "engineering director" etc. If you use the word "engineer" in your job title or company's website for services provided, the engineering association will come after you unless you are licensed and have a Permit to Practice.
Many times engineers have non-engineers for managers who push around the noobs and direct them to skirt around safety and exploit loopholes, to rush a project and save money.
When
the bridge collapses, they go on a head hunt for whoever reviewed and stamped the drawings, as they should. But when the planes crash, nothing was really done for Boeing's corruption in the 737 Max's engineering.
Engineers are still the fall guy for corrupt leadership or a corporation that will not let them design/test properly. Tesla seems to be able to make mistakes with autonomous vehicle S/W and nobody goes to jail there.
So registering engineers will not really guarantee safe buildings, bridges, products etc. Defective/corner-cutting manufacturing can also generate unsafe products.
Registration would be a small step forward at great expense to engineers. But still zero consequences for the CEO and other usual crooks.