Green’s Dictionary of Slang

(Aus.) a drink favoured by down-and-out alcoholics, comprising methylated spirts and copper sulphate (bluestone).

1924		Richmond River Herald (NSW) 20 May 2/2: A witness [...] who was shot on leaving a sly groggery, was under cross-examination: How many rums did you have? — A good few; I have been known to have 40 rums and then go. home. (Laughter.); What! Banyan rums ? — Yes,.
1930		Truth (Perth) 4 May 3/: DAYS OF ‘BANYAN RUM’ / [...] / MEN CRAZED BY SLY-GROG [...] The recipe for Banyan rum was said to contain a liberal supply of methylated spirits, whilst a bullock’s liver, boiled down, was used for coloring purposes.
1946		Longreach Leader (Qld) 14 June 14/4: A somewhat similar concoction is known in Queensland as Banyan Rum — so called after the Banyan River which runs through Tully. Mecca of tough drinkers in our Far North. Instead of boot polish you can mix bluestone (copper sulphate) with your methylated spirits.