All [UK] ballot papers are numbered and, with a court order it is possible, legally, to find out how anyone voted!

I'm led to believe that the Australian version of secret ballots was known, early on in California, as the Australian Ballot. The version now used in Oz requires you to present yourself to the polling booth, where you are asked your name (and, if necessary to eliminate ambiguities, your address) by an attendant. Your name on the electoral roll is marked and you are issued with the relevant ballot paper(s), each of which is not numbered but is marked by the attendant with their own signed initials. You mark your ballot paper with your votes (we have preferential voting and, for the Senate, preferential voting and proportional representation) and plonk the marked ballot paper(s) in the ballot box under the supervision of an attendant who makes sure it is folded, can't be retrieved, and isn't removed from the booth. Ballot papers damaged after being issued but prior to being put in the box can be replaced (by destroying the first and reissuing another by the same process) but I've never seen it happen.

During counting the number of papers in the booth's ballot boxes is tallied against the number of voters attending that booth but there is no way that any individual's paper can be connected to the person. The number of Informal votes is often commented on by psephologists as an indicator of how dis/satisfied the electorate may be with the general picture.

Cheers, Rowan