I think my government is taking a wait and see approach. They'll probably try to deal with this as a capital gains tax, which is triggered when you move value out of the STEEM. This is generally how tax works for other assets: CGT when it the assets is converted.
The situation with STEEM is a little more subtle; would transfers between STEEM, SP or SBD trigger a capital gains tax? Dunno. And, unlike other cryptos there is a labour element that goes into creating value on STEEM. I wonder how I could claim expenses for that to offset the CGT? Claim a grammarly subscription? Research fees?
Either way, if STEEM goes huge, then the blockchain is public. Once a govt associates your user account with a tax-payer-id then they can very easily figure out how much that citizen made on STEEM and send a tax demand automatically.