The eureka flag was originally flown by striking miners in the Eureka Rebellion 1854, from memory its the first flag to feature the southern cross, it wasnt adopted by the colonies until victoria adopted its current flag later on

Colonists largely being christian is a big part of my issue, being in a mostly christian state I was a bit blind to the significance of the cross until recently, but look at this shit

A depiction of The Southern Cross in Mark Twain's 1897 travelogue, "Following the Equator"