Green’s Dictionary of Slang

1. (N.Z.) foolish; self-deluded; incompetent [= wank adj.].

1988		McGill Dict. of Kiwi Sl. 98/1: septic silly, self-deluded or incompetent, aka ‘wanker’ or wankery; from rhyming slang ‘septic tank/wank’.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. (also seppo) American [septic (tank) n.¹ (1)].

1983		J. Byrell (con. 1959) Up the Cross 66: The Cross was crawling with Septic Tank sailors.
2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 She has an accent you could swear was pure septic tank.
2003		www.thepantsman.com 🌐 There are endless lines of airheaded seppo tarts holidaying for summer vacation who LOVE the antipodean.