The Australian Bureau of Statistics is warning people not to post pictures online that reveal the unique barcode on their same-sex marriage survey forms amid concerns it could jeopardise individual results.

Marriage postal surveys have begun to arrive in Australian mailboxes, but people are already taking to social media to upload images of their forms, some even posting photos with their personal details and unique barcodes in clear view.

The barcode, which appears in the bottom right corner of the survey form, is used to link a specific form to an eligible voting Australian.

The purpose of it is to ensure no one can respond to the survey more than once.