I don't think a buy out is possible or necesssary, because...

I don't think a buy out is possible or necesssary, because PanAus has the whip hand. Consider who would put up the money to buy out HIG's 20%. Let's say that there is a cashed up white knight with $200 mil. What would they get? They get a dominant partner which has its own agenda, and does not have to listen to the minor partner (except for the proviso below). So if PanAust wants to spend billions on a dam and electricity system, then the white knight has to finance 20% of that build, which at best won't be returning any income for five or more years, and take at least a decade or so to repay $6 billion of debt. I mean to say, who would jump into that frying pan? So, who else could buy out the 20%? The only possibility is PanAus, but why would they? Already they have shown that they have control of the development of Frieda. What they really need is funds to carry out their big plan, so why would they spend more money on buying HIG's 20%.
My opinion is that the only way forward is for HIG and PanAus to work together. If they don't, HIG has just one option (the proviso), and that is to explain to PNG (local communities, Government, NGOs and environmental groups) their reasons for not wanting a massive start-up development with a risky dam/hydro. If the JV can't modify PanAus's plan, HIG needs to show PanAus that it can stall or stop PanAus's agenda by reminding all in PNG of the ongoing disaster caused by a massive, risky mine at OkTedi. Hopefully HIG can channel PanAus towards the small-scale, less risky, quicker startup that they originally planned. If they can, I see a really big opportunity for all.