Also a defence under Australian law.
> The statement is the speaker's opinion.
Also a defence under Australian law, as long as the speech falls under the concept of "fair comment and criticism", which is the same rule the US follows (albeit interpreted quite differently).
> it must proven that it was spoken with malice
NOT a defence under US law, unless the plaintiff is a public figure, which is a category defined quite narrowly in the US.
> So defamation in the US is close to unenforceable.
There are a lot of libel suits in the US, many of them are won by the plaintiffs.
The difference in law is less than you seem to think, and what differences exist are mainly on the extent that opinions ("I think Joe is an asshole") should be protected; both countries agree that factual claims ("Joe murdered his wife") are not. And offhand, it's hardly clear that Australia had drawn the line in the right place.
See, eg:
https://www.nytimes.com/2019/03/05/opinion/australia-defamat...
https://www.theguardian.com/media/2018/nov/30/your-right-to-...
https://www.smh.com.au/national/australia-s-defamation-laws-...
No but...
Valid defenses include: the statement is true. The statement is the speaker's opinion. And even if those fail, it must proven that it was spoken with malice. So defamation in the US is close to unenforceable.