Green’s Dictionary of Slang

1. (Aus.) well-off, affluent.

1845		T. Mccombie Arabin 241: They had a pretty little farm, and were well in .
1858		‘A. Pendragon’ Queen of the South 170: I shall see Jem Clayeter, who’s well in, and wont let a friend be stuck for a hundred or so.
1891		‘Rolf Boldrewood’ Sydney-side Saxon 1: He’s a well-in squatter that took up runs or bought them cheap before free selection.
1901		Bulletin (Sydney) 14 Dec. Red Page/3: If a person fails ‘he has gone bung,’ and if he is well off ‘he is pretty well in.’.
1924		G.H. Lawson Dict. of Aus. Words And Terms 🌐 WELL IN — Affluent; wealthy.
1941		Baker Popular Dict. Aus. Sl.
1955		N. Pulliam I Travelled a Lonely Land (1957) 241/2: well in – well off financially.

2. popular, secure, entrenched.

1900

Bulletin (Sydney) 6 Oct. 12/4: [A]fter several futile attempts to make ’em ‘part,’ his reverence (who is ‘well-in’) said: ‘Weel, gentlemen, ah’ll lend it te ye mysel’ at 8 per cent.’ – and he did.

3. successfully ingratiated, on the way to a successful seduction.

2003

McGill Reed Dict. of N.Z. Sl. 228: well in Successful in an enterprise or project.