Green’s Dictionary of Slang

1. (Aus.) in gambling, betting on credit.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxvi 4/2: murray cod: On the nod, to bet on credit, on the Murray.
1977		Australian Mag. 23 July 1: A punter [...] who bets ‘on the murray cod’ (the nod) walked into City Tattersalls on Monday settling day carrying $240,000 in cash [GAW4].
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 36: Murray Cod On the nod (bet on credit).
1999		Complete Online Gaming Guide No. 9 24 Nov. 🌐 Betting on the nod – or betting ‘on the Murray’, as it is known in racetrack rhyming slang – is the preferred way of business for most professional punters.
2002		Sydney Morning Herald 22 Nov. 🌐 There has been an influx of punting, too, from some of the desperadoes. That’s the sign of hard times when people are tying to punt their way out of trouble. [...] ‘A lot of them are on the Murray Cod [nod or credit] and are hard to find after the event.’.

2. (Aus. prison) with an unspoken arrangement.

1990

Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Murray cod. An unspoken arrangement. Rhyming slang for ‘on the nod’.