Sounds like they wanted to try and make an example of him for leaking information outside of the ATO. Pretty brutal response, they do really need to strengthen whistle blower protections more.

I think it’s because he took/leaked confidential documents belonging to the ATO. This isn’t covered by the public information disclosure laws. You do not need to collect evidence, that’s up to the investigators, you are encouraged to provide your own records eg notes, journals.