Green’s Dictionary of Slang

(Aus.) a drinker in a public house.

1900		Bulletin (Sydney) 13 Oct. 14/2: Clancy, the cattle-man, struck the pub. and asked a ‘stool-pigeon’ that was in the bar to have a drink. Then they had another, and another, and the stool-pigeon bore up every time, but when the fifth drink came on, Clancy’s cattle-education came to his rescue. He had a ‘Jimmy Woodser,’ and remarked to the stool-pigeon, ‘Old man, I’m going to wean you.’.
1902		Bulletin (Sydney) 6 Sept. 29/1: There was four or five stool-pigeons there – a dry an’ bumming crowd, / I shouted for the lot of them; I shouted very loud.