Green’s Dictionary of Slang

1. (Aus.) the female breasts.

1944		L. Glassop We Were the Rats 50: I looked at the maternal expanse above her waist. [...] Spike would have called it ‘a decent sort of an upper deck.’.
1957		D. Niland Call Me When the Cross Turns Over (1958) 148: I like the cut of you, your shape. I like your upper-deck, the way you swing your bottom.
1966		Trimble 5000 Adult Sex Words and Phrases.

2. see decky n.