The big 5 generally refers to:
Christianity; Judaism; Islam; Buddhism; Hinduism
The top belief systems in Australia are:
Christianity – 52.2%
No Religion – 30.1%
Unspecified – 9.6%
Muslim– 2.4%
Buddhism – 2.4%
References:
Central Intelligence Agency (2021). CIA - The World Factbook. Retrieved 06 Feb, 2021, from https://www.cia.gov/the-world-factbook/countries/australia/