Firearms ownership aside, I do not believe that the "right" to protest (a form of expression), is an inalienable or "basic human right". It should be IMO, but isn't.
While somewhat ancient in concept (a couple thousand years - maybe), freedom of expression is a modern philosophical idea in practice. In this country, the application of "Freedom of Speech" was born from tyranny, and why the First Amendment to our Constitution (and in support of that, the 2nd Amendment as well) - as part of the "Bill of Rights".
The UK, which purports to have as its constitution (which there is no single formal document for) a series of documents (body of law) that prescribe it, it is still all subject to Parliament, not the people directly per se, as in the US. In other words, similar law could be enacted in UK - which no doubt would be amidst protest. The UK's adoption of the European Convention (as an EU member), does however bind them to a very specific freedom of expression/speech description (with many exceptions) as a "Human Right". Note that this was adopted only in the late 1990s (not in the late 18th century as it was in the US).
History aside, I don't know that Australia's Constitution has a formal, general, explicit statement regarding freedom of speech. I am not too familiar with it, but don't believe they do. The exception - IIRC - is what they refer to as "political speech".
I am not defending the Australian's treatment of protestors philosophically, but the law (of a given society), often irrational creature that she is, is their law, therefore not really of my concern. I have no plans to go there and protest anything.
In fairness, there are constraints to the interpretation of our own "Freedom of Speech", which do not allow for violent protest (incitement of riot or other anarchy), screaming "fire" (false emergency) in a somewhat confined space with a sizable number of people present, attending university courses without clothing (silly example), and what have you.
Regarding the firearm buyback in Oz, I am not familiar with the crime statistics prior to, during, or shortly following this process - so I am in no position to comment. As for "success", what's the metric? What I do know, that for Australia at the time, it was very expensive (which is a societal cost).
Regardless gents, these are the laws of countries in which we don't all reside. What they choose to do, is up to that society (in this case - a state) or nation-state. Being judgmental about their choices (on a moto forum - lol), is unlikely to result in change.