Green’s Dictionary of Slang

1. a scout or spy.

1825		C.M. Westmacott Eng. Spy vi 162: Dick’s a trump and no telegraph – up to every frisk, and down to every move of the domini, thoroughbred and no want of courage .
1901		‘Rolf Boldrewood’ In Bad Company 39: He’s a jolly telegraph, ain’t he? Why, Joe Kearney the sprinter could have run all the way and beat him, hands down.

2. (Aus.) a member of a bushranging gang whose task is to keep the others informed of the whereabouts of potential victims or efforts to capture them.

1888

‘Rolf Boldrewood’ Robbery Under Arms (1922) 173: We sent Warrigal out to meet one of our telegraphs [...] and to bring us any information he could pick up.

3. a network of gossip and rumour that brings news (often inaccurate) before the official sources.

1866		Tumut and Adelong Times (NSW) 1 Jan. 2/3: They approached to within one hundred yards of the camp unobserved, and then it was apparent that the ‘telegraph’ had done its work .
1883		J. Keane On Blue Water 202: It has always appeared quite wonderful to me how the ‘telegraph,’ ‘white mouse,’ or ‘little bird’ gets about such ships, but it is a fact that everything that is said and done among the men is known aft nearly as soon as it is forward.