Green’s Dictionary of Slang

1. (Aus.) opium.

1898		Bulletin (Sydney) 1 Oct. 14/3: A few more W.Q. [i.e. West Queensland] slang words...Opium ‘twang’, a Chinaman a ‘canary’ [etc.].
1904		Truth (Brisbane) 10 Apr. 5/6: [T]he fearful curse of OPIUM SMOKING, or, to use the local vernacular, ‘twang sucking’.
1966		Baker Aus. Lang. (2nd edn) 157: Opium was once known as twang in bush slang.

2. in fig. use, a fantasy; nonsense.

2011

J.J. DeCeglie Drawing Dead [ebook] I shipped her off with [...] some bullshit about how I’d call her in a couple of days. It was complete twang.