The Australian Atheist Foundation is running a campaign encouraging people who are no longer religious to answer "no religion" on their census form on August 9.

The foundation's website, CensusNoReligion.org.au, encourages people who identify as atheist, humanist or rationalist to tick the "No religion" box rather than writing their preferred identifier in the blank space below the question.

The foundation also encourages people who are no longer religious to mark the "no religion" option rather than the religion they were brought up in.

(They are also urging people not to fill in their religion as "jedi" or anything like that, because that throws off the number of atheists in Australia.)

"Often the transfer of your tax dollars to religious organisations is justified on the basis of the Census results. Also special concessions and exemptions are given including the right to discriminate against some groups," the website states.

But religious organisations fear that faith groups could be marginalised if too many people who don't belong to an organised religion choose the "no religion" option.