Every now and again a business has to change Banks and you are left to figure out what the ABA file format should be and what the different fields mean.

Especially annoying is when you have set it up and have been running for years and the meaning has been lost over time.

Thankfully the problem has been encountered and documented before:
http://ddkonline.blogspot.com/2009/01/aba-bank-payment-file-format-australian.html

Strange that the format isn't published by the standards organization: http://www.bankers.asn.au/

Looking at the Format it mentions an "Australia Payments Clearing Association" http://www.apca.com.au/