Green’s Dictionary of Slang

septic (tank) n.¹

1. (also seppo) an American [= Yank n. (2)].

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxix 4/4: septic tank: Yank.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 43: Septic Tank Yank.
1985		Tracks (Aus.) Aug. 5/1: I agree – Bruce Springsteen is an overrated, bum-wriggling Seppo.
1992		R.G. Barrett White Shoes 186: You’ll like Mrs Llivac too. She’s a seppo.
1993		B. Moore Lex. of Cadet Lang. 328: septic tank rhyming slang for ‘Yank’, any American.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 191: [T]he two chuffed Septics got the best pacer in the world for a sonnet.
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 70: All of a sudden one of the Septics is back.
2000		Hartford Courant (CT) sect. D 5 Sept. 27/3: G’Day from Down Under [...] You septic tanks always have been a snag short of a barbie, when it comes to other cultures.
2005		P. Howard Curious Incident of the Dog in the Nightdress 42: Iran, or Iraq, or wherever the Septics bombed the shit out of.
2006		Choda Boy 57 at Urbandictionary.com 10 Aug. 🌐 When your leggies are disappearing for 9 an over, or you’ve kicked 4 behinds and 2 out-on-the-full for the day, or your pitching’s been racked for 3 homers in 1 innings (if you’re a Seppo), mate, you are having an absolute Barry.
2012		(con. 1943) G.S. Manson Coorparoo Blues [ebook] ‘You been hangin’ out with seppoes too much’.
2015		I. Welsh Decent Ride 31: Now ah’ve goat another Septic on the mobby, that fucker Ronnie.
2020		D. Whish-Wilson Shore Leave 44: ‘You helping the Septics?’ [ibid.] 97: ‘The Septic Tank can leave’.

2. a bank.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxix 4/4: septic tank: [...] bank.
1982		G. Newbold Big Huey 253: Septic tank (n) Bank’.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 43: Septic Tank [...] bank.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 162/1: septic tank n. 1 the bank .

3. a tank.

1983

R. Aven-Bray Ridgey-Didge Oz Jack Lang 43: Septic Tank [...] tank.

In compounds

Seppo-land (n.)

America.

1996

Tracks (Aus.) June 12/2: My employer needs me in Seppo-land at the same time.