Green’s Dictionary of Slang

(Aus./N.Z.) an informer, a tale-teller.

1895		Coburg Leader 30 Mar. 4/4: There is a dobber down East who will be getting Simmons(ed) for smoking a pipe.
1968		D. Ireland Chantic Bird 149: Girls are great dobbers.
1981		P. Barton Bastards I Have Known 26: Estelle was the real dobber.
1999		G. Seal Lingo 184: Likewise our aversion to dobbing in and dobbers (now called whistleblowers and officially approved) and our espousal of the fair go.
2009		P. Temple Truth 111: Unless they got a dobber, they had nowhere to go and so the media unit fed a stream of rubbish about positive identifications in the hope that one of the pricks in the group who didn’t actually kick the man [...] would dob in those who did.
2016		D. Whish-Wilson Old Scores [ebook] There was always a chance a neighbourhood-watch dobber or someone with an eye on the hundred-thousand bounty would put him in.