The big 5 generally refers to:
Christianity; Judaism; Islam; Buddhism; Hinduism
The top religious belief systems in the World are:
Christianity – 2 321M adherents
Islam – 1 554M adherents
Hinduism – 955M adherents
Buddhism – 494M adherents
Sikhism – 25M adherents
Judaism - 14.5M adherents
The top belief systems in Australia are:
Christianity – 12.7M adherents
No Religion – 3.8M non-adherents
Buddhism – 419k adherents
Islam – 340k adherents
Hinduism – 148k adherents
References:
Australian Bureau of Statistics (2011). ABS: Census of population and housing – 2006 Data. Retrieved 10 Oct, 2011, from http://www.abs.gov.au/websitedbs/censushome.nsf/home/Census
Central Intelligence Agency (2011). CIA - The World Factbook. Retrieved 10 Oct, 2011, from https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html