Green’s Dictionary of Slang

(Aus.) one who works outside a carnival, sideshow, strip club etc, touting the pleasures inside and pulling in the customers.

1941		K. Tennant Battlers 143: The ampster’s is an easy job. He stands in the front row of the listening crowd registering interest and enthusiasm while the showman ‘spruiks’. [...] The ampster rushes eagerly over to the ticket-window and says: ‘Right-oh, mister, I’ll have a ticket.’ He pretends to pass his money over, and is handed a ticket. His brother-ampsters form into an impatient queue behind him and file into the tent at the head of the multitude, who, like sheep, will follow the leader, but will not be the first to pay their money. If it were not for the ampsters, there would be no audience.
1945		Baker Aus. Lang. 138: Terms for various sharpers, tricksters and others who live by their wits: spieler, [...] amster (or ampster).
1957		D. Niland Call Me when the Cross Turns Over (1958) 101: Barbie, playing ampster, went up and bought a bottle [...] Others followed to buy.
1967		Parade (Aus.) Oct. 8/3: Niland roamed Australia working in jobs that ranged from circus hand to ampster and gee-man in travelling boxing shows [OED].
1975		H. Porter Extra 244: A shady Soho club patronised by dips, amsters, off-duty prostitutes.