Green’s Dictionary of Slang

(Aus.) an informer.

1946		News (Adelaide) 22 Nov. 2/3: Two classes of prisoners get a rough time in gaol-sex offenders and ‘fizzers,’ gaol slang tor police informers. ‘Fizzers’ are shunned by other prisoners and have to be on their guard all the time.
1950		S.J. Baker in Sun. Herald (Sydney) 8 June 9/1: Among other people well know to the constabulary are the ‘top off,’ ‘fizzgig,’ ‘fizzer,’ and ‘shelf,’ all names given to informers.
1953		Baker Aus. Speaks v. 124: A stool pigeon or informer, otherwise known as a fizgig, fizzer, shelf and topoff.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 28: Fizzer Same as phizzgig.
1999		G. Seal Lingo 50: Equally Australian are a number of terms for the person who keeps guard for those involved in criminal activities, including [...] the crickety-sounding long-stopper and fizz-gig, or fizzer now used widely to mean a police informer.