Green’s Dictionary of Slang

lobster n.2

[ety. unknown; ? pun on idea of being ‘boiled alive’; note rock lobster under rock n.]

1. (Aus.) a $20 note.

[Aus](con. 1943) G.S. Manson Coorparoo Blues [ebook] She produced a wad of lobsters and counted out 10 of them.
[Aus]T. Peacock More You Bet 66: The ‘$20 note’ is known a ‘red-back’, after the spider, or as a ‘lobster’, or a ‘lobby’ for short, due to colour of a $20 note resembling that of a boiled lobster.
[Aus]N. Cummins Tales of the Honey Badger [ebook] [They were] $1 a pair [...] So I laid out two lobsters and got meself 40 pairs.
[Aus]P. Papathanasiou Stoning 162: [H]e should’ve offered less, a lobster, a twenty.

2. (Aus. prison) a 20-year jail sentence.

[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Lobster. Twenty year sentence. Becoming obsolete.