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The Australian-English expression heifer-paddock designates:
– (literally): an enclosure in which calves are isolated from their mothers until weaned;
– (figuratively, humorously and offensively): a girls’ boarding-school.

—Cf. also the expression heifer-dust.

In heifer-paddock, the use of the noun heifer to derogatorily designate a girl or woman is similar to that of the noun cow—as explained in the following from A Dictionary of Slang, Jargon & Cant. Embracing English, American, and Anglo-Indian Slang, Pidgin English, Tinkers’ Jargon and other irregular Phraseology (Edinburgh: The Ballantyne Press, 1889), by Albert Barrère and Charles Godfrey Leland [Vol. 1, page 457, column 2]:

Heifer paddock, (Australian), a ladies’ school. The derivation from heifer, a young cow—cow being a slang word for a woman—is obvious.

The earliest literal use of the expression heifer-paddock that I have found is from The Emigrant’s Guide; Or Ten Years’ Practical Experience in Australia (London: W. S. Orr & Co., 1845), by the Reverend David MacKenzie (floruit 1845-1852) [chapter 19: Cattle, pages 119 & 120]:

Stable, barn, storehouse, an additional wheat paddock, a large heifer paddock, (into which you can put your calves which are fit to be weaned,) and a small paddock, near the huts, for your riding horses and working bullocks. […]
Without a weaning or heifer-paddock, you will be obliged to allow your calves to continue sucking their mothers for a whole year, to the serious injury of the latter; and you will also be obliged to allow your heifers to have calves, as in such circumstances they often have, before they are twenty months old. The result will be, that many of them die in calving, and that, if you allow the survivors to rear their calves, the growth of the mother is stunted, and the calf will be a disgrace to your herd. Nothing tends more to deteriorate a herd than to allow the heifers to breed at too early an age. They ought to be three years before they are sent to the bull.

The earliest figurative use of the expression heifer-paddock that I have found is from a conversation about marriage, in Australian Life: Black and White (London: Chapman and Hall, 1885), by the Australian author Rosa Campbell Praed (née Murray-Prior – 1851-1935) [chapter 2, page 50]:
—Note: This conversation takes place at the narrator’s house, in a remote inland district of Australia; the narrator has just explained: “Naraigin, Eurogan, and Milungera formed an unequal triangle lying westward. The last named was two days’ journey from us, and was the furthest station in the district. Beyond, the country was unexplored.”:

“The fact is, my dear Murray,” he added, “the cattle hereabouts are too scattered, you can’t inspect them properly. Next year I shall look over a heifer-paddock in Sydney and take my pick.”
N.B.—Heifer-paddock in Australian slang means a ladies’ school.

The U.S. novelist and humorist Mark Twain (Samuel Langhorne Clemens – 1835-1910) mentioned the figurative use of the expression heifer-paddock in Following the Equator: A Journey Around the World (Hartford (Connecticut): American Publishing Co. – New York: Doubleday & McLure Co. – 1897) [chapter 22, page 221]:

Australia has a slang of its own. This is a matter of course. The vast cattle and sheep industries, the strange aspects of the country, and the strange native animals, brute and human, are matters which would naturally breed a local slang. I have notes of this slang somewhere, but at the moment I can call to mind only a few of the words and phrases. They are expressive ones. The wide, sterile, unpeopled deserts have created eloquent phrases like “No Man’s Land” and the “Never-never Country.” Also this felicitous form: “She lives in the Never-never Country”—that is, she is an old maid. And this one is not without merit: “heifer-paddock”—young ladies’ seminary. […]
And then there is the immortal “My word!” We must import it. “M-y word!” In cold print it is the equivalent of our “Ger-reat Cæsar!” but spoken with the proper Australian unction and fervency, it is worth six of it for grace and charm and expressiveness. Our form is rude and explosive; it is not suited to the drawing-room or the heifer-paddock; but “M-y word!” is, and is music to the ear, too, when the utterer knows how to say it.