You can also go to a number of pick-up locations in capital cities and regional areas, or phone the ABS's info line.
Ordering a new form will render your original form invalid.
How long will this campaign last?
The ABS wants everyone to post their forms back by October 27, with a hard deadline of November 7.
Both the ‘Yes’ and ‘No’ campaigns are expected to run advertising right through to the end of the survey.
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When will we know the result?
The ABS will publish the results on its website on November 15.
The Chief Statistician will also give an analysis of the survey’s integrity.
What happens next?
The postal vote itself cannot legalise same-sex marriage.
In the event of a 'Yes' vote, the government says it will allow a private member's bill to be introduced to the House of Representatives in the final parliamentary sitting fortnight of 2017 (which begins in the last week of November).
Members of parliament would then be given a free vote.
The bill is widely expected to pass in this case. Parliamentarians will not be bound by the results of the postal survey, but many, including some conservatives who oppose same-sex marriage, have promised to respect the decision of the people even if it goes against their personal beliefs.
If the people vote "No" in the postal ballot, Prime Minister Malcolm Turnbull said no bill would proceed.
Labor leader Bill Shorten has promised to introduce a bill to legalise same-sex marriage in his first 100 days as prime minister if he wins the next election.
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