Is there any relation between circular quay the secret bridge shit you were talking about and the place in Sydney or were they just like really not feeling creative that day and like fuck it it’s an place where people cross the water call it the same thing (could I google this, yea but I feel like your answer will be more fun)
The location in Sydney is exactly the reason! It was thought that if any information was leaked about movements around the Somme river (which you couldn’t mention by name in your letters), hopefully people would assume ‘Circular Quay’ was talking about the quay in Sydney, and not a shed on the river bank full of portable bridge
The pontoon bridge was notoriously narrow and difficult to traverse, which happened at night with little light. The British were notorious for trying to take wagons across which were usually as-wide as the bridge itself and often overloaded. The ANZACs tipped many wagons into the river because the wait to get across was quite literally deadly.