Green’s Dictionary of Slang

(Aus. und.) a clandestine informant, working within an institution, e.g. the police.

1994		Mollen Report 8: [A]n anonymous survey of Internal Affairs investigations with the assistance of an Internal Affairs ‘insider’.
2013		D. Whish-Wilson Zero at the Bone [ebook] [W]hat was interesting was that Gus Riley’s insider in the police force and the Sydney cop invited to Perth for a shoplifting spree had one person in common – Detective Inspector Ben Hogan.