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.
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).
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...
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.