After looking into the file formats for bank transactions the other day, I was wishing that Westpac had some form of public API to call. When it struck me that one of my own family works on the Westpac DeskBank support team.
So I emailed off asking for information. I got the reply that yes the file format was documented. But no public access was available.
In case anyone is looking at the file format and wanting to understand it better, the support team seem to give the document away, but it can also now be found here.