Trade agreements (eg: ChAFTA in this case) typically contain clauses which restrict the ability to pass laws that directly harm the interests of one countries investments in the other.

But in any case, transmission grid operators are natural monopolies that are usually heavily regulated and can’t simply charge whatever they want. I’m sure South Australia’s grid is no different.

Well, based on that every utility should be sold off to a foreign country since only then are you prevented global trade laws and treaties from passing laws (or even failing to enforce civil fines?) that harm their interests.

Frankly, I'm doubtful this is a case of actual enforcement of global trade rules, it's pure economic power. That works until it doesn't, and even the purchasers of these profitable "rights" know this. That's why they want to get their money out quickly, but once the local government has turned against you it's rarely worth fighting even for the largest funds.

Trade agreements usually contain provisions for dispute resolution. Back before 2016 the big boogeyman was ISDS (investor-state dispute settlement).

If a state made a bad deal they can either renegotiate it (eg. negotiate in a fee to cancel the whole contract) or try for arbitration (and the tribunal might set a fee for cancellation), or assert their sovereignty.

Of course with China everything that reaches political circles becomes a power-play, but at least in the first two cases they are less motivated to respond in kind (ie. levy tariffs or fuck with the business of Australian companies in China).

Sure, but there's no dispute here that I'm aware of.

Turns out the South Australia grid isn't really owned by China anyway - they have a 46.6% stake, with the other 53.4% controlling interest now held by Australian Utilities, a consortium of Australian infrastructure and pension funds.

https://realassets.ipe.com/news/australian-utilities-trust-e...

There will be be repercussions from both China and global trade organizations as applicable - although that may indeed be worth it in the end. In addition the country will lose a bit of credibility with investors putting at risk investment in other areas where the government might "change its mind" like natural resources. This is hard to quantify but has an impact in aggregate.

Ontario, Canada sold one of the countries most important highways back in the 90's to investors for 100 years but at least the owners were still Canadians.

It depends on how they do it. If they simply renegotiate or go through various arbitration/adjudication processes, then the impact can be small. (Of course if they don't accept the outcome of these processes, then it'll be worse.)

That said, on this level it's almost 100% politics. There are many ways to get out of these unfair deals made in times of desperation, the question is - like you said - are they worth it or not.