Green’s Dictionary of Slang

(Aus.) a pickpocket’s accomplice who pushes up the arm of the victim to facilitate access to their wallet; thus push-up mob, a gang of pickpockets specializing in this; at the push-up, working as a pickpocket; thus push up, v.

1919		V. Marshall World of Living Dead (1969) 129: He acts as chief amongst his ‘push-up’ and ‘break’ men, associates skilled in their way, but unpossessed of his dexterity.
1923		Smith’s Wkly (Sydney) 2 June 21/1: ‘He could at least “cop” from me and push up. If there was a “stink,” he'd have the goods on him, and I should worry.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 8: Push-up mob: A pickpocket gang who work the tram or bus queues, one of them obstructing the platform whilst others jostles passengers.
1938		F.D. Sharpe Sharpe of Flying Squad 15: Pick-pockets are known as ‘Wizzers’ or ‘The Push Up Mob’. [Ibid.] 332: push up (the) : Picking pockets.
1989		J. Morton Lowspeak 116: To be at push up – pickpocketing.