One of the questions that pops up often is which flag did the Australian Flying Corps use as the national flag? Was it the British Union Flag (Union Jack), the Blue Ensign (the current national flag) or the Red Ensign? The RAAF history page has an interesting photo:
British heraldry was adopted by Australia in the 18th and 19th centuries. Britain divided its flags up into the Union flag, the Blue Ensign, the Red Ensign and the White Ensign. The Union flag became the national flag. The Blue Ensign denoted government bodies and buildings. The Red Ensign was a civil flag most commonly used for the Merchant Marine while the White Ensign was reserved for the Navy.
Australia followed the same path until the 1950s when the Flag Act came into being and the Australian Flag became the Blue Ensign.
Prior to that government bodies, such as states, used the Blue Ensign. Australian merchants on the sea flew the Red Ensign and the Australian navy used the British White Ensign until the Vietnam war when Britain was concerned that the Australian use of the flag might cause confusion that Britian was involved in the Vietnam war.
Before 1953 it was assumed that in Australia the national flag was the Union Flag, the government flag was the defaced Blue Ensign and the civil flag was the defaced Red Ensign. In reality a mix of all three were used with the Red Ensign being the most popular. Canada used a Red Ensign as their national flag until replacing it with the maple Leaf flag.
So we see in World War I a mix of the Union Flag, Blue Ensign and Red Ensign being used. The photo in the intro is a good example of this.
