The Halloween trick-or-treating comes from an old British custom called "A-Souling." There's a pretty good write-up here:
Souling is a tradition that goes all the way back to Pagan times, but has evolved into the 'trick-or-treating' practice that we know and love today
www.history.co.uk
Since Australia was originally a British colony, it doesn't surprise me that the colonists brought the custom with them.
Oh, right! Now it makes more sense.

I remember reading about soul cakes in, of all books,
The Folklore of Discworld (where it's part of the discussion about the
Soul Cake Duck, a parody of both the Souling Cake and the Easter Bunny. The Soul Cake Duck lays eggs sweeter than the usual duck eggs, and is also connected with a tradition called 'trickle-treating' or 'treacle-treating').
Souling was one in a family of traditions involving performing song, music, or small theatrical skits in return for food or money. On November 2nd, All Soul’s Day, 16th-century northern and western English revelers would go “a-souling” door to door. In return for their performance, their neighbors would give them soul cakes. The gift of a soul cake had three benefits: charity for the poor performers, the good work of giving charity, and the redemption of souls from purgatory, hence
soul cakes.
One "souling song" went like this:
“Soul, soul for a souling cake
I pray you, missis, for a souling cake;
Apple or pear, plum or cherry
Anything good to make us merry!
Up with your kettles and down with your pans,
Give us an answer and we'll be gone.
Little Jack, Jack sat on his gate
Crying for butter to butter his cake
One for St. Peter, two for St. Paul
Three for the man that made us all.”
(Shropshire:
Bye-Gones Relating to Wales & the Border Country (1889–1890), 253).
Soul cakes, by the way, were not as soft as cakes are now. Rather, they were dense and biscuit-like, with sweet spices like nutmeg.
Here is a possible recipe.
November 2nd is also the second day of El Dia de Los Muertos, the Mexican Day of the Dead.
In Poland, November 1st is the day when relatives visit cemeteries and bring chrysanthemums to decorate the graves. November 2nd is a more sombre day – a day to stay home and think of loved ones, perhaps looking through albums of photographs.