There's no argument in that class actions can be raised against the Gov, and that the Gov is held to higher standards in such litigation through the Model Litigant Guidelines under the Legal Services Directions (and should respect such guidelines and treat plaintiffs accordingly).
However, the PFAS cases stem from an argument on whether the Gov has breached its duty of care or has been negligent in one way or another. The argument is that the Gov should not have allowed the continued use of a harmful substance and should have to answer to the harm that has occurred (and will continue to occur) because of it.
How do you start a class action against the Gov for not giving money to a particular business that isn't entitled to it? Although anything is arguable in law, why does the Gov have to give VA money, or support people/organisations who consciously invested into an organisation that was already making a loss?
Investments inherently carry risk, and the Gov did provide support to the airlines through underwriting the domestic network, subsiding certain fees and excise, and even subsidising the workforce through JobKeeper. I would argue that just because VA was in a precarious position before the pandemic does not make the Gov liable when their actions so far have arguably been in the public good.