Fading Truth - in countries like Australia and the UK, a commission like this has powers to force people to appear and testify, as well as to demand any and all documents related to the inquiry. The penalty for failing to do this can be imprisonment and/or fines. There is no legal way out of this, beyond some very specific exemptions. It's more akin to Congressional investigative hearings than a civil court case.
The records come from two requests made in February 2015 by this commission. They evidently were held by the Australian branch.