Epic sign is epic. I believe it is now known as Adelaide Arcade.
In August 1885, Gays Arcade was announced in course of construction with a 100 foot frontage to Twin Street and a depth of 119 feet to Adelaide Arcade. N W Trudgeon built Gays Arcade for 11,000 pounds. The architect was James Cumming who also was the architect for the Australian Mutual Provident Building in King William Street.
More info – https://adelaidearcade.com.au/history/