Scientology describes itself as the world’s fastest-growing religion but the organisation's numbers in NSW are dwindling with the church reliant on overseas devotees, defectors claim.

The organisation has properties worth millions across the CBD, Chatswood and Dundas and in 2017, doubled its national revenue to nearly $40 million, the latest published financial records show. But it is struggling to find new recruits in Sydney, defector Paul Schofield said.

"Australian recruits have dwindled to virtually zero as media coverage and access to the Internet has ensured the truth about the cult is well-known here," Mr Schofield said.

The Church of Scientology headquarters in Chatswood.

The church, which was founded by American science fiction writer L. Ron Hubbard, also has a history of altering its membership figures, the defector who spent 30 years in the church rising to become an ordained minister of Scientology said.