[caf]

In reality you have a choice. You can either rank all, or if you are lazier you can just select your favourite candidate and then your first choice candidate's preferences will be used instead.

This is not true. In the elections for the Australian House of Representatives (single-member electorates, comparable to the mayoral election under discussion where there is a single winner), you must number every box for your vote to count.

In elections for the Australian Senate (multiple member electorates) you don't have to number every box, but you have a choice: you can either vote "below the line" for individual candidates, in which case you must number at least 6 boxes for your vote to count (the ballot paper advises you to number at least 12); or you can vote "above the line" for groups of candidates (in which case you only have to number one, but the ballot paper advises you to number at least 6). A vote "above the line" for a group is equivalent to numbering the candidates in that group in order from top to bottom, but it doesn't imply a vote for a candidate in any other group.

The voting systems in Australian states and territories vary from the above; some of them do allow only numbering a single candidate in single-member elections ("Optional Preferential Voting" or OPV) and some still have group-ticket voting in multi-member elections where you can assign your vote to the preference ticket submitted by a candidate (though hopefully the last few jurisdictions to still have this will be getting rid of it, because it is being gamed).