My guess is that the ANZ legal department checked exactly the form of the contract that Opes clients signed to ensure that Opes had the right to use the shares as collateral for the $650 million dollars loan facility. ANZ probably cross checked with ASX to ensure that there would be no hindrance should they need to execute their right to sell the shares should they need to foreclose on the loan. That would be a minimum needed before the bean counters at ANZ would deliver a cent to Opes. If you are asking about the ethics of ANZ's behavior in this then I should remind you that we are talking about a bank here.
