Let's be clear here, the letter from BankWest will not mention the word abuse so they don't need to justify it. They will say they are closing your account down because the T&Cs allow them to do so.That might be true, but I don't believe it's reasonable for Bankwest to argue abuse when all the account holder has done was doing the minimum to meet the spend requirement, wouldn't you agree that's similar to a credit card company closing your account and withhold the bonus points for just spending the minimum amount to get the bonus points and no more?
I would definitely take it to FOS if Bankwest goes down this path and I hope many others will as well.
Underlying their decision to close your account may well be the way you have been using the account but you can only take to FOS what they do say, not what they don't. If you look back at this thread this has been going on a long time and I haven't seen one report of FOS success because the T&Cs do indeed give them the ability to close your account and don't require them to give a reason.