Much of the musing comes from narrator Bloodworth, a thief who spends much of his time watching the seals, penguins and "whalefish", and trying to decide whether he's in heaven or hell.
"Whatever flow into me flow into them too," Bloodworth thinks, "even if they ain't got no soul to collect it."
As the foursome's futures dwindle to a pinprick they learn of one another's painful pasts, confront their sins and debate how best to survive the freezing conditions. There's an amusing irony that these are law-breakers forced to create their own rules to live peacefully - fat chance of that. Self-appointed leader Slangam insists it's hard work that will see them through, whereas Bloodworth is a self-described lazybones who increasingly isolates himself from the group. Then there's know-it-all Gargantua, who believes himself superior to the others, and Toper, an Irish drunk with a dark secret.
Even the humble potato comes to symbolise their opposing views on life. Should they toil tirelessly, growing as many as possible? Or celebrate by eating the occasional green one, which may have hallucinogenic properties?
"On one level it's a straightforward tale about people," says Randall. "On another it's a bunch of types duking it out according to what they think about right and wrong, and who should do what and whether they're innocent or guilty."
The island soon becomes a symbol of freedom and imprisonment, idyll and despair. And although these are tough felons they're not immune to fear brought on by superstitions about what may lie in the depths of the forest.
"I added a gothic thread so it didn't seem pastoral," says Randall. "It's gruesome in parts because I didn't want it to be seen as men romping in flowery fields."
It's also quite a comedy. Out of boredom, Bloodworth has a go at meditating, or as he puts it, trying to "efface" himself.
"It turn out pretty hard to rub yerself out when yer only got yerself to do it with."
Randall says she's had plenty of practice at creating truthful characters but is in the dark as to where their wisdom comes from. "I sit in a chair and stare at a computer screen for hours and hours. And I live with a philosopher. My husband is always correcting my thinking. He doesn't know when he gives input but I start to think about what he thinks."
In her previous novel, Hokitika Town - which, like Eleanor Catton's Booker-winning The Luminaries, is set in the gold rush era - Maori protagonist Halfie spoke in a sort of pidgin language. Some readers thought that was "weirdly racist", says Randall, but he was a plucky and intelligent character who simply hadn't been brought up in an English-speaking world. Likewise, the four characters in The Bright Side Of My Condition are "salty sailors" who have been through the school of hard knocks and, because of that, talk that way.
"Maybe my laziness turn inside out and show itself a virtue," says Bloodworth. "Maybe I'm jes a whole lot less trouble when I'm sitting on my arse."
"I'm very concerned with how characters speak," says Randall. "When I was researching I looked at the Robert Hughes book, The Fatal Shore, and there were quotes from people in jail that struck me as weird English. I didn't understand the rules of their dialect so I thought I'd make my own one up."
This adds a poetry and a rhythm to the novel. But perhaps the great strength of Randall's writing is her ability to question modern problems within a historical setting. She has much to say about society's endless pursuit of happiness.
The Robinson Crusoe quote, from which the book gets its name, hints at Bloodworth's ups and downs. There are moments when a "toad mood" comes to visit, and the others fear for his sanity.
Randall touched on similar themes in her second novel, The Curative (2001), in which a man incarcerated in a 19th-century mental asylum undergoes barbaric "cures", challenging readers to consider who the real crazies were. It was joint runner-up for a Montana Book Award. Her What Happen Then, Mr Bones? (2004) and The Crocus Hour (2008) were finalists for the same award. Many of Randall's books are set in the past, where the characters often confront psychological woes, or, in the case of the central family in What Happen Then, Mr Bones?, achievements and misadventures in medical science.
Like most writers, she's fascinated by motive. But Bloodworth, she says, is not mentally ill. "He gets a bit down, but so what? He says he likes the melancholy violin. He gets too much of the bright lights."
She has no preference for historical stories, as the history itself is not the point; they simply allow her the freedom to play with ideas. "I don't want my characters to go off to work and come home and argue with the family or whatever. I want them to seem real, but they're vehicles for what I want to say. Which is why I'm a writer. If you can't make the dummy speak it's pointless."
The Bright Side of My Condition (Penguin $30) is out now.
