Pressure is mounting on the federal government and opposition to agree to an inquiry into the Church of Scientology, amid claims they are shielding it from abuse allegations for political reasons.

Social commentator Phillip Adams has accused the two major political parties of turning a blind eye to the issue after they joined forces to vote down a parliamentary inquiry into the church's, and other charities', tax-free status last week.

Speaking at the Global Atheist Convention in Melbourne, Mr Adams said Scientology was a pseudo-religion that shouldn't enjoy charity status in Australia.

He said Australia should follow the example of the United Kingdom, where a strict public-benefit test applies.

"To hear those idiots in the Senate talking about Scientology as a religion when it's a racket is ludicrous," he said at what organisers called the largest gathering ever held in Australia under the banner of atheism.

"It just makes you realise how wonderfully protected the group actually is."

Mr Adams told the audience of atheists, sceptics, humanists, feminists and secularists that Scientology was "a dreadful outfit of excesses and cruelties".

"And here in Australia we are continuing to turn a blind eye to it."

Independent senator Nick Xenophon, who called for the inquiry, will try to launch another one this week focusing on allegations against the church, including claims of forced abortions, imprisonment in boot camps and separation of families.

Former Australian Democrats leader Lyn Allison, who is also speaking at the three-day conference, says she doubts the inquiry will get up.

Politicians were scared the challenge to tax-free status would spread to other religions, the former senator said.

"Whilst individually they (the government and coalition) probably don't like Scientology, they do not want to open up Pandora's box," she told AAP.

Ms Allison, who now works for a number of human rights groups, was critical of the federal government's position that an inquiry was not needed because the tax-free status of charities was already being looked at through ongoing tax inquiries.

"We've had tax inquiries in the past, and the findings were that there was no case for revisiting the tax exemptions for charities. Any new inquiry is unlikely to be any different," Ms Allison said.

Fellow presenter Max Wallace, who has written extensively about tax exemption for religious groups in Australia, says tens of millions of dollars in taxpayers' money is handed to the churches each year.

"Exemption from taxation for churches is a 17th century idea that has no bearing on the 21st century," he told the conference.

But he noted one anomaly in Australia, the UFO-based religion known as Raelianism.

"The Raelians believe that there are extraterrestrial beings in another galaxy. The tax office decided that because those extraterrestrial beings were material, and not supernatural, it didn't fall into the definition of religion for tax exemption."

The conference, including talks by popular science writer and atheist Richard Dawkins and controversial ethicist Peter Singer, continues. All 2500 tickets to the March 12-14 event sold out in advance.