Green’s Dictionary of Slang

(Aus.) a police officer, usu. male.

1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 89: JOHN DUNNS: Aust. thieves policemen.
1904		Sydney Sportsman (Surry Hills, NSW) 13 Apr. 3/6: [F]our harmless individuals who ‘happened to be there' when the John Dunns ‘raided’ one little plots [sic] in Market-street.
1914		Bulletin (Sydney) 24 Dec. 44/2: The policeman swallows his furtively, and then says ‘Good-night’ and leaves. / ‘’Andy ter be in with the John Dunns,’ says the youth with the foxy face. [...] / ‘Drinks widger to-night, sloughs yer in ter-morrer.’.
1919		Smith’s Wkly (Sydney) 28 June 14/3: Emily Soldene claimed that the Melbourne cant phrase for a policeman — ‘John Dunn’ [...] ‘was pure Collingwood’ for ‘gens d’armes’.
1941		Baker Popular Dict. Aus. Sl. 39: John Dunns, policemen.