A sample copy of the same-sex marriage postal survey form has been published online.
As you can see from the image below, the question "should the law be changed to allow same-sex couples to marry?" is accompanied by a 'yes' and 'no' box.
Instructions below state that only one box can be ticked, and note that each paper has a barcode. If there is no barcode or both boxes are ticked, the vote may be invalid.