The phrase bush telegraph denotes any rapid informal network by which information, rumour, gossip, etc., is spread.

This phrase originally referred, in Australian English, to any chain of communications by which bushrangers were warned of police movements. (The noun bushranger denoted an escaped convict who took refuge in the outback.)

The earliest occurrence of the phrase bush telegraph that I have found is from an article about bushrangers, published in The Freeman’s Journal (Sydney, New South Wales) of Saturday 28^th February 1863:

We find that a great number of the inhabitants are in constant and active communication with the gangs of bushrangers infesting the country; who are allowed to meet and plot without molestation; whose plans are, in all probability, assisted; and, certainly, whose escape is facilitated and whose capture is prevented. If a trooper happens to be in unpleasant proximity to the gentleman for whom he is in search, a young squatter or farmer takes horse and proceeds to the spot where the victim to a tyrannical law is concealed, to warn him of his danger; or if the police are upon his track, the bush telegraph is set to work to give him time to secure his retreat.

Published in The Sydney Morning Herald (Sydney, New South Wales) of Saturday 18^th July 1863, the following describes how the bush telegraph was organised:

The discovery of gold broke up the old system of mutual dependence, and drafted off from regular occupations a large number of persons, who, calling themselves gold-diggers, are responsible to none, and are under no effective surveillance. The opening of gold-fields in sparsely occupied neighbourhoods brought at once the object of universal covetousness into an article of export. Formerly, probably, the inhabitants of these places, though of the lower class, were simply loose and disorderly. But the opportunities presented to them converted them into a clan of robbers. Their own community supplied the machinery by which these depredations could be planned and executed, property disposed of, and felons concealed. Their command of horses presented no ground for suspicion, and their familiarity with all the byways of the country gave them an advantage over any strange constabulary however active or skilful they might be. They were enabled to establish a bush telegraph which, by signals known only to the initiated, could secure the more active members of the commonwealth of thieves from pressing danger. We have heard of one contrivance which will remind our readers of signals of the most ancient times. A boy upon a horse is dispatched to a certain place for some trifling object. He is not trusted with the secret of which he is really the bearer, but as he passes in a certain way, or upon a particular horse, the bushrangers understand that the road is or not clear for their operations—that the constables are present, or that they are gone. Thus, by various signs and tokens, the people who were entitled to be unsuspected are really the most effective abettors of robbery and pillage.
[…]
It would be difficult for any constabulary to move about the country in force without their functions being known and their movements telegraphed. No doubt it is desirable that they should not wear a uniform which may be recognised at a distance, but what can possibly prevent the communication of their movements in a country where they are the subject of universal conversation transmitted by travellers from town to town. It appears, however, that the police do not wear their uniform when employed in the bush but only as patrols, and it must be recollected that passengers would be liable to no small alarm at the approach of two or three well armed [sic] horsemen if they were not distinguishable as public servants.

The phrase bush telegraph also denoted a person warning bushrangers of police movements—as in the following from The Sydney Morning Herald (Sydney, New South Wales) of Tuesday 11^th August 1863:

On Thursday afternoon, one of the most daring outrages that have yet been committed by the scoundrels who infest our highways, was perpetrated within four miles of Carcoar, close to the Waterholes. By the mail, which left Carcoar at mid-day, three prisoners, named Thomas Morris, Charles Green, and James Burke, were conveyed under the custody of Superintendent Morisett and constables Grainger, Merrin, and Sutton. The prisoners are supposed to be bush telegraphs, and it was to rescue them that the attack we are about to chronicle was made.

The phrase bush telegraph soon came to be also used as a verb meaning to warn bushrangers of police movements—as in the following from This Day’s News: Latest Telegrams, published in The Adelaide Express (Adelaide, South Australia) of Monday 27^th March 1865 (Dan Morgan (c.1830-1865) was a notorious bushranger):

Two men accused of bush telegraphing in connection with Morgan, have been brought up at Albury.

The earliest occurrence that I have found of bush telegraph, used in the extended sense of any rapid informal network by which information, rumour, gossip, etc., is spread, is from The Sydney Morning Herald (Sydney, New South Wales) of Tuesday 6^th October 1868:

The old complaints reach us from New Zealand of the insincerity of the Friendly Natives. It is not indeed supposed that they are wanting in courage more than the rest of their race, but they are not hearty in the service, and their adherence to the British Government is purchased. There is a story told of the assistance rendered on a memorable occasion to the European forces by the friendly natives. They advanced with great fortitude and fired with great earnestness; but there was an understanding between them and the enemy that when they loaded their guns they should drop their bullets. Thus after the fight a great quantity of lead was found, not in the enemy, but on the ground! There can be little doubt either that the plans of the British are disclosed by their native allies, or that a “bush telegraph” of a very effective kind is readily established.