Green’s Dictionary of Slang

(Aus.) of quality,excellently, very well; of size, extremely.

1910		Bulletin (Sydney) 1 Dec. 32/1: As for beds, in one place I camped in a half-built pub; in others, I’ve rolled myself up in my rug under a tree and ‘slept boshter,’ and on two occasions when it was too wet for outdoor sleeping I went down to the shed called the railway station and ‘bunked’ on the seats. There’s no hardship in it.
1912		Benalla Standard (Vic.) 13 Dec. 4/1: Almost immediately he had a bite – not a mosquito or a fly but a ‘boshter’ big cod.
1924		Land (Sydney) 14 Mar. 6/6: Larry assoomcd a joodishal air and looked solemn like as he took a squint at the birds – boshter big coves they wus, too.
1934		Blackwoods Mag. CCXXXV 458: Billo reckoned we’d got to find some dead straight bloke, who’d crack hardy, know the ropes to pull and spout his piece and beef it out all boshter.