Green’s Dictionary of Slang

(Aus., Northern Territory) a person who is newly arrived and thus inexperienced.

1931		‘William Hatfield’ Sheepmates 118: Hallett took charge of the three ‘parcel post’ men and showed them a bunk where they could deposit their belongings.
1946		W.E. Harney North of23° 23: Young lads bursting with romance and itching to be out in the wide open spaces, would sign on in the cities to work on far inland stations. ‘Parcel post men’ they called them; that is, they were labelled and addressed to a certain place, and travelled as a parcel does in the mail.
1951		E. Hill Territory 431: ‘Parcel Post’ was influence—a station manager, or a jackeroo from the cities, who might be the boss’s nephew [...] never lived down the stigma if he got a job, not by reputation or work, but by ‘knowing the heads.’ ‘He come up by parcel post,’ they still say. ‘He don’t know nothin’.’.
1959		Bulletin 22 Apr. 16: You parcel-post b-! I pay twenty-five pound a head for the best night-horses on the road to be spoiled by a b- like you!