Winton uses idiom and dialect in this scene to insert local flavor and create a casual, humorous tone as Quick spends Saturday nights having sex with Lucy Wentworth in his truck:
Saturday nights he sees Lucy Wentworth, or various moonstruck parts of her, in the cab of the truck, parked up some dwindling road behind a decrepit grove of salmon gums.
You’ve got a huge whanger, she says. That’s what I like about you, Quick. A head like that, it’d be eligible to vote.
Lucy is flattering Quick here, which he clearly enjoys. She’s comparing the head of his “huge whanger” (his penis) to a person’s actual head. The idiom “eligible to vote” is a playful exaggeration; she’s suggesting that Quick’s penis is so large that it could qualify as its own person. Its head is so big that it must contain a brain, which would make it an Australian “eligible to vote.” Their sexual banter is light and fun, and neither takes the other person too seriously. Their relationship is short-lived, as they don't have much in common, but they are still able to enjoy one another's company.
The word “whanger” comes from Australian slang and adds to the informality of this exchange. It’s a rough, unserious word that sounds strange in a sexual context. It’s not usually an erotic descriptor for a penis, and that’s part of what makes this scene feel almost like banter between friends. The use of dialect here keeps the scene grounded in local language. Lucy’s casual vulgarity gives the moment a sense of realism, as does Winton’s hyper-local imagery of “a decrepit grove of salmon gums.”