Given they have specifically referred to their half-year accounts, it may be that they are in dispute with their auditors who are threatening to qualify the accounts in some way. If this is the case the company has no choice but to suspend the stock until the issue is clarified one way or the other and a clear statement can be made to the market.
$3B in liquidity is not just sitting in the bank account waiting to be spent. It includes cash received for which services have yet to be delivered, undrawn credit facilities even though the funds are committed to capex etc. It is likely that this figure will be substantially reduced by the change in credit rating which means the $600M of credit card payments Qantas receives in advance of the flight being taken will now only be released when the flight is taken just like most other airlines.