‘a shag on a rock’: meaning and origin

The Australian-English phrase a shag on a rock is used as a type of the isolated, deprived or exposed.

This phrase refers to a shag (i.e., a cormorant) perched alone on a rock.

The phrase a shag on a rock is used, for example, in Cadet call-out for officers and staff, published in the Torres News (Thursday Island, Queensland) of Monday 29th April 2013:

Four top Navy brass flew into the Torres Strait recently for a meeting hosted by the Torres Shire Council.
The officers, from the Royal Australian Navy (RAN) and Australian Navy Cadets (ANC), came to the meeting with one objective in mind: to recruit the key instructors and officers needed to reinvigorate Thursday Island’s TS Carpentaria Navy Cadets.
[…]
ANC National Commander John Gill said cadet staff would be given the support they need.
“The staff will not be left here like a shag on a rock, they will receive ongoing support and training,” Capt. Gill said.
“There are lots of mums and dads with no military training who make great cadet trainers, given the right support.”

These are, in chronological order, the earliest occurrences of the phrase a shag on a rock that I have found:

1-: From Bush Yarns.—No. 4: Pat Fagan’s Journey Overland to Timor, by ‘Fubbs’, published in The Sun, and New South Wales Independent Press (Sydney, New South Wales) of Saturday 18th February 1843:

We came in sight of a fine field of ripe corn, just down below us, in a broad level plain at the foot of the mountain, and the sun shining so beautifully on it. My heart leaped within me, to see that we were near human beings like ourselves again; and, says I to Joe, “come down here and we’ll pass for free men, and ask for work in reaping the corn, its [sic] quite ripe—there’s the cove himself, in the field, and now or never, Joe—come on.” “Just step down yourself, Pat,” says he, “and I’ll sit up here, on the top of the hill, watching you, and if it’s all right just give me the office, and I’ll come down to you.” Well, down I went, and, with my hands in my small-clothes, I sauntered, carelessly, along by the side of the fence, near which he was. “Fine day, sir,” says I. “Yes,” says he. “That’s a beautiful field of corn (God bless it),” says I. “Yes, pretty fair,” says he—eyeing me suspiciously. “Do you want any hands to reap it?” says I. “Why,” says he, “do you want work?” “Yes, sir” says I, “me and my mate, that you see up there (and I pointed up to Joe Brady, who was sitting silent and solitary, with his thumbs under his ears, like a shag on a rock) are looking for a job.”

2-: From Australian Note-Book, in Impressions of Australia Felix, during Four Years’ Residence in that Colony; Notes of a Voyage Round the World; Australian Poems, &c. (London: Longman, Brown, Green, and Longmans, 1845), by the British poet Richard Howitt (1799-1869):

The common people are not destitute of what Wordsworth * calls “the poetry of common speech,” many of their similes being very forcibly and naturally drawn from objects familiarly in sight, and quite Australian.
“Poor as a bandicoot,” “Miserable as a shag on a rock,” &c.; these and others I very frequently heard them make use of.

* This refers to the British poet William Wordsworth (1770-1850).

3-: From Police Intelligence, published in the Gippsland Guardian (Port Albert, Victoria) of Friday 13th February 1863:

D. T. McKenzie v. Mrs. William Wood.
Defendant was charged with having used at Port Albert, on the 2nd February, abusive and obscene language calculated to produce a breach of the peace.
Mr. McKenzie sworn, deposed: I am a publican, and reside at Port Albert. On the 2nd February I heard Mrs. Wood, opposite my house and inside the house, make use of indecent language. She called my wife a b——y w——e and called myself a thief and said I kept a w——e shop, and that she would have me like a shag on a rock in less than a month.

4-: From Mudgee Quarter Sessions, published in The Sydney Morning Herald (Sydney, New South Wales) of Friday 8th July 1864:

Joseph Lant, senior.—Attempt to scab sheep.
[…]
The count charged the prisoner with wilfully communicating scab, on the 5th March last, to 1000 sheep, the property of Mr. Orr, of Garrawilla, Coonabarabran.
[…]
Senior constable Ward, deposed: Knew the defendant; had seen him at Coonabarabran bringing shirts for his son; he then said he would make Mr. Orr b——y glad to leave the district in less than six months; he had £300 to spend on him, and would leave him as poor as a shag on a rock; as poor as he came into the country.