Compulsory voting does not imply a politically engaged populace. I believe the majority of voters do not enjoy the requirement to attend on polling day, and treat the whole affair like supporting a football team. Except they care a lot less about the election results than their football team. Mandatory voting probably has the effect of protecting the two major parties more than anything else.
There was a surge in minor parties in the Senate about a decade ago. In the (very large) Senate ballot you were able to vote for a single party, and their official preferences registered with the Electoral Commission were automatically applied to the entire form. Minor parties with similar policies were obviously preferencing each other, and preferencing major parties very low. (The whole point of a minor party, sometimes a single issue party, is a significant deviation from the uncontested policy agreed upon by both major parties). So the process was changed and now the voter has to manually nominate at least 6 parties on the Senate ballot form. The effect of this is even the motivated voter with interest in the policies of a minor party cannot usually name 6 that they prefer over the 2 major parties, and so one (if not both) will end up getting a reasonably high vote preference when the previous scheme would have seen them near the bottom. This further secured the position of the two major parties.
There was a surge in minor parties in the Senate about a decade ago. In the (very large) Senate ballot you were able to vote for a single party, and their official preferences registered with the Electoral Commission were automatically applied to the entire form. Minor parties with similar policies were obviously preferencing each other, and preferencing major parties very low. (The whole point of a minor party, sometimes a single issue party, is a significant deviation from the uncontested policy agreed upon by both major parties). So the process was changed and now the voter has to manually nominate at least 6 parties on the Senate ballot form. The effect of this is even the motivated voter with interest in the policies of a minor party cannot usually name 6 that they prefer over the 2 major parties, and so one (if not both) will end up getting a reasonably high vote preference when the previous scheme would have seen them near the bottom. This further secured the position of the two major parties.