Sydney's traditional northern Bible belt is losing its religion as suburbs in the south-west emerge as the city's most devout.
The share of people with no religion in the Bible belt districts of Ku-ring-gai, Hornsby Shire and The Hills Shire has climbed sharply over the past decade, an analysis of the 2016 census shows.
In the Ku-ring-gai council area the population share with no religion has risen from 16.3 per cent in 2006 to 31 per cent in 2016 and is now above the national average.
Ku-ring-gai has traditionally been seen as Sydney's Anglican heartland but the proportion in the district affiliated with that denomination has slumped from 27.1 per cent to 18.8 per cent over the past 10 years.
In The Hills Shire, home of the Hillsong mega church, the share of people with no religion has swelled from one in eight in 2006 to one in five.
Meanwhile, Sydney's south-west has consolidated its position as the city's believer belt. The highest proportion of those who affiliate themselves with a religion is in a band of suburbs stretching through the council areas of Liverpool, Fairfield, Cumberland and Canterbury-Bankstown.
Suburb-level census data provided to Fairfax Media by the Bureau of Statistics shows neighbourhoods with the highest ratio of Christians were in the city's west and south-west including Grasmere (82.3 per cent), Theresa Park (81.1 per cent), Abbotsbury (81 per cent) and Horsley Park (79.6 per cent). In those suburbs the most common denomination was Catholic followed by Anglican.
Some suburbs in Sydney's south-west also had a significant share of Muslim adherents including Lakemba (59.2 per cent), South Granville (49 per cent) and Old Guildford (45.9 per cent).
Nationally, the proportion of the population opting for "no religion" on census night has risen from 18.7 per cent to 29.6 over the past decade.
Associate Professor Ruth Powell, an Australian Catholic University researcher on religion in the community, said the national trend to set aside religious identity was especially strong among people of Anglo ancestry.
This has been a factor in the decline of religious affiliation in Sydney's northern suburbs Bible belt.
"When you think about the previous traditional Bible belt, I assume you have a higher proportions of people with Anglo ancestry, therefore that area has been more impacted than others by the trend for people to tick the no religion box," she said.
In the Parramatta council area, which borders The Hills Shire and hosts the historic St John's Anglican Cathedral, Hindus now outnumber Anglicans.
The 2016 census showed Greater Sydney to be more devout than any other capital - 24.6 per cent of the city's residents described themselves as having no religion, 5 percentage points lower than the national figure.
Even so, Sydney has some of the nation's least religious suburbs, especially in the city's inner west. In Darlington, 60.6 per cent reported having no religion on census night. Other suburbs with a high share identifying with no religion were Erskineville (54.8 per cent), Newtown (53.3 per cent) and Enmore 52.3 per cent).
Sydney had 10 suburbs (with more than 100 people) where half the population or more said they had no religion.
Hobart was Australia's least religious capital, with close to 40 per cent of the population reporting no religion in the 2016 census, followed by Canberra (36.2 per cent) and Adelaide (35.6 per cent).