[LINK] ABS could find out how you vote.

[Forename Surname]

https://marriagesurvey.abs.gov.au/privacy-policy#barcodes

The ABS says of the vertical barcodes on your survey formS:
"...The 'sequence number' also enables the insertion machine to identify and group together the correct instruction letter and survey form to be put in an individual envelope for each eligible Australian..."
I. e.  your survey form and the form with your name and address are linked together by this barcode.  Thus anyone with access to the original forms could very easily see which ballot paper belongs to whom.
Someone  m i g h t  also be able to use the big barcode at the bottom of your survey to figure out who you are, but I suppose the A. B. S. has probably made this quite a bit harder.

Credit due to Paul Templeton from the  E. F. A. Privacy mailing-list (though any erroneous theories or crack-pot conspiracies are entirely my own concoction).
Privacy at lists.efa.org.au 
http://lists.efa.org.au/mailman/listinfo/privacy

Sorry all for not saying much you probably didn't already know,
Reubs Newbs.