“Our members have been unable to obtain any formal clarification on the reasons for closure, except for references to policy or risk. Just what policies or risks these are have not been specified,” Ron Tucker, ex-chairman of industry body The Australian Digital Currency Commerce Association, complained back in September.
http://venturebeat.com/2016/03/18/fear-or-greed-why-are-australian-banks-cutting-off-bitcoin-businesses/
The references to risk are actually valid. Accounts related to bitcoin definitely pose a higher risk of money laundering. This would mean enhanced monitoring and higher costs for the banks. So it is a business call about whether they want the account or not.