The bush telegraph; the grapevine. Also mulga. (1899 —) . K.S.Prichard The troops've had it all by mulga. They've heard too (1950). M.Durack Local gossip flourished through a word-of-mouth medium referred to as 'the bushman's mulga wire' (1983).
[From Austral. mulga the outback, from earlier sense, type of acacia tree, from Aboriginal (Yuwaalaraay) malga + wire telegraph.]