Green’s Dictionary of Slang

(Aus./N.Z.) to treat (others or oneself) to a drink; thus spotting n., occasional drinking; also as n., a drink.

1924		Lawrence & Skinner Boy in Bush 90: T’ be down on y’ uppers [...] whilst y’ slog t’ the nearest pub t’cadge a beer spot.
1926		J. Devanny Lenore Divine 58: He knew that most girls, quite ‘nice girls’, ‘spotted’ themselves nowadays, and he had never before met a girl who shuddered at the mere smell of spirits or beer [DNZE].
1930		Northern Advocate 11 July 7: [heading] The ‘Spotting’ Habit. Young people of 16 and over... thought it bright and clever to appear ‘three sheets in the wind’ at country dances and made a practice of running out after every dance to motor cars to ‘have a spot’ [DNZE].
1947		‘A.P. Gaskell’ ‘The Big Game’ All Part of the Game (1978) 189: [1978 note] ‘spots’. Any alcoholic drinks, but usually spirits. The verbal forms ‘to spot’ and ‘spotting’ were common.
1950		A.L. Cherrill Story of a N.Z. Sheep Farm 73: If it weren’t for you, Pa wouldn’t have got into the way of spotting the way he has.
1987		N. Hartley Swagger on Doorstep 180: Mother believed dancing was worldly if not sinful, and she was frightened that we might start ‘spotting’. Mother’s twin fears were carnal sin and having a ‘spot’ [DNZE].