As the articles say, he has been under investigation for a while for his bitcoin mining work (which as far as I can tell is not under dispute). The Australian government has decreed that all bitcoin "investments" should be taxed for capital gains [1] so he could be up for a huge tax bill on his paper "profits" even if he was just mining back in the early days.
[1] https://www.ato.gov.au/General/Gen/Tax-treatment-of-crypto-c...