It seems to me that an essential part of a liberal democracy should be the citizen’s legal right to decide whether or not to vote. The denial of that right is an affront to democracy.

Okay, except, you absolutely do still have the right, and choice, not to vote in Australia. In practice, the only actions that are actually compulsory are showing up to a polling centre, having your name marked off the roll, and placing your ballots in the ballot boxes. There is nothing stopping you from walking straight over and chucking them in without marking them at all. Parties pay attention to the informal vote totals as well.