Green’s Dictionary of Slang

(Aus.) a salary; a profit.

1991		R.G. Barrett Between the Devlin 10: [T]hey’re [i.e. corrupt police] only out to get whatever earn they can while they’re in’.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 29: F.C. Standish took over from Captain (Sir Charles) MacMahon as chief commissioner of police in Victoria with an annual earn of £1200.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 299: [A] taller and heavier D. Sutherland is making a bit of an earn singing with a rockie group called the Thunderbirds.
2006		B. Matthews Intractable [ebook] He was looking for an earn.