A project to get atheists to tick the "no religion" box in the upcoming census has faced a backlash from anti-Islam campaigners, who say that it would ensure Islam becomes Australia's dominant religion.

The Atheists Foundation of Australia has been trying to get people to stop marking their birth religion down on next month's census (which is basically a compulsory survey of the entire Australian population) if they no longer practice that religion.

The foundation's President, Kylie Sturgess, told Hack that the group was not telling people to abandon Christianity, but wanted people who weren't religious to fess up to it.

"If you're a Christian, awesome! Tick that box," Kylie says. "Just try and tick the truth."

We want to see what kind of country we actually are."

But the campaign has faced backlash from some people who are opposed to Islam.

"Do not leave the religion section blank, please," an email doing the rounds says. "Be sure to tick Christian (or your upbringing faith). Every Muslim will tick their box. 10,000 Australians will leave it blank then wonder why a mosque is built in their neighbourhood!!"

"On census night fill in what you where [sic] born under Catholic or Christian otherwise the Muslims will claim Australia because you can bet you're [sic] balls they will fill in muslim," another campaigner wrote on AFA's Facebook page.

"Though I am an atheist, I'll be putting down the religion I was born into. I won't be letting Islam take the upper hand. This is really important," another woman wrote.

Islam not that popular in Australia

In 2011, just over 476,000 people (2.4 per cent of the population) identified their religion as Islam, making it the 7th most popular in Australia.

The biggest religious group is Western Catholic (27.4 per cent) followed by No Religion (23.9 per cent). But only 3 percent, or just under 59,000 people, identified specifically as atheists.

Since 2001, there's been an, ahem, underground movement trying to get Jedi Knights listed as an official religion. People can write whatever they like as their religion in the "other" category, and the ABS says Jedi Knights will be classified as "not defined".

After the 2011 census, the AFA campaigned heavily to get "No Religion" moved to the top of the census question that asks for religious information.

They were successful; in this year's census, "No Religion" will be the first option.

Other countries that put "No Religion" up the top noted a bounce in figures, Kylie says.

Like our cousins across the ditch in New Zealand. Secular organisations like the AFA say the number of people who recorded having no religion jumped from 35 per cent to 42 per cent when the box was at the top of the list.

Kylie denies that the AFA's campaign is about spreading atheism, but she acknowledges that there has been "a growing amount of curiosity" about what being an atheist means.

She says religion has played a role in how politicians make laws, like the Coalition's decision to hold a plebiscite, and some politicians' opposition to abortion.

The census offers information that can go on to inform lawmaking. For example, it can determine if an area needs a new road or school based on the data collected from the local population.

This year's census will be held on Tuesday August 9, and is compulsory. The ABS says if you don't fill it out, you could be fined.