This is an expression to indicate whacking off (hence whack-off-ia). The bank part comes into the picture because every time you whack off, you usually deposit something.
John: Hey Paul, you spider monkey, have you been wanking all night again??
Paul: Hush John! Don't say it loudly. But yeah I made a large deposit at Whackoffia Bank last night.
A quaint but usefull Australian term of excitement. Mostly used by Australian men over the age of 80 and only after a good lie down before 3pm. Etymology uncertain but was in common usage before Twitter.