Australia is supposed to be a secular society, but the Atheist Foundation of Australia says the nation's biggest outdoor advertising company has refused to run its advertisements.

One of the humorous messages the foundation hoped to put on the back of buses was, "Sleep in on Sunday mornings".

But the foundation says Australia's biggest outdoor advertising company, APN Outdoor, had a problem with it.

Atheist Foundation president David Nicholls told the Religion Report on ABC Radio National that the contentious slogan was one of a number which had been proposed for the $16,000 advertising campaign.

"We started off with 'Atheism - because there is no credible evidence', we put that to the bus companies, they didn't like that and they said the wording wasn't to their acceptance," he said.

"And then we changed that to 'Celebrate reason' and thought we'd make it a bit comical - 'Sleep in on Sunday mornings'. But they refused that also.

"The end conversation I had was I asked why we were refused and my answer to that was, 'well we have to refer this to our legal department', and chappie hung up," he said.

"Nearly immediately after that he rang back with a message saying listen this is all finished, we're not putting the signs on."

"I'm disappointed but not overly surprised. Religion has a very great hold on societies, even democratic societies, and in fact it has too great a hold in democratic societies."

Advertising abroad

Atheists overseas have had more success.

In the United States for example there is a cheeky advertisement on Washington buses: "Why believe in a God? Just be good for goodness' sake."

"In Australia, religion started off having a lot of breaks way back when we were starting our society, and those breaks have continued throughout," Mr Nicholls said.

"Religions are tax exempt, they receive council breaks, they're responsible for a $100 million chaplaincy program, tax payers are paying for religious schools, and 'celebrate reason' is a very appropriate message to be putting forward today."

The World Today sought comment from APN Outdoor's marketing general manager Paul McBeth. But he has not returned our calls.

Mr Nicholls suggests it is a bit much to knock back a slogan questioning God, when a group which does believe in God has been allowed to run its advertising.

"In Adelaide I believe, in the recent past, the APN buses had all across the back of a bus, or some buses, the message 'John 3:16', which is the most famous biblical passage, that God so loved the world that he gave his only son da da da," he said.

"Now atheists find that offensive, and there's lots of atheism in Australia, but we wouldn't prohibit its displaying."

Greg Clarke, the director of the Centre for Public Christianity, says freedom of expression should be favoured as often as possible.

"As long as it's done with a level of civility that means we actually do engage with the issues, rather than just fighting with each other," he said.

Mr Clarke says he is surprised the atheist advertising campaign has been knocked back in a secular society like Australia.

"It probably does indicate that religion and God are a really hot topic at the moment, and so you've got to be careful," he said.

"I actually think that they should be a hot topic, and if you can stop and contemplate God while a bus rushes past then your days all the better for it."

-Based on a report by Brendan Trembath for The World Today