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.