Their mother's death is the first of many tragic existential ironies the sisters will face. Most, as is so often characteristic with depictions of the first world war, examine the value of human life in a world that routinely sacrifices it to abstract notions of the greater good: patriotism, empire, victory, honour and duty.

This familiar trope is given poignant new twists. Take the medical setting. Dazed after their first encounter with dead and dying young soldiers, Naomi and Sally overhear a heated conversation between the two surgeons aboard their floating hospital, the Archimedes. The resilient Dr Fellowes is trying to persuade his colleague, the shellshocked and undertrained Dr Hookes, to carry on after Hookes confesses to cutting the femoral artery of an injured soldier. Fellowes attempts to calm him down: "Are you worried about the nurses seeing, or the mistake? A doctor is always a peril to people, dear Ginger. As well as a rescuer. How many have you saved in the past few days? Ask yourself that."

The Durances make for tantalising central protagonists: "both the girls were aloof and looked similar - dark and rather tall." Few indeed can tell them apart, and it is the standoffish sisters' tragedy that, after disaster eventually strikes one of them in Europe, no one home in Australia is sure which one died: "It was said around the valley that the two Durance girls went off but just the one bothered to come back."

This knowingly flippant opening sentence kickstarts the central narrative drive of the novel: which of the two Durances will fail to endure?

Like Fellowes and Hookes, the Durances are altered by their first encounter with the aftermath of warfare. Naomi had always been the fearless pioneer: she became a nurse before Sally, left home before Sally and joined up before Sally. Yet, given the responsibility of diagnosing the gravity of a soldier's wounds, she falls quietly to pieces. These are picked up by her little sister Sally, who finds previously undiscovered reserves of coolness under pressure.

Gender offers one of many unexpected perspectives on the conflict. While women assert their capable authority over matters of life and death, men are newly vulnerable thanks to heroic wounds, embarrassing dysentery, or shameful gonorrhoea and syphilis. These are Keneally's polar extremes, however. In between, he portrays a vast array of people and different emotional reactions. While many of the nurses keep their heads, others stagger under the weight of the violence they witness.

There are also many examples of male bravery and stoicism, but even this is nuanced. Naomi's suitor, Robbie Shaw, braves an injury with greater fortitude than he does his eventual rejection at her hands. The problem is not Robbie's wound, but the fact that Naomi loves someone else: an unstuffy, Quaker lieutenant, Kiernan. It is characteristic of both her slightly reckless courage, and Keneally's world-turned-upside-down, that Naomi declares herself first to the man she loves. "It is quite a changed world indeed, [Kiernan] told her, in which women have the courage to say what must be said."

Robbie is understandably hurt: love may be more equal in war, but it is still not fair.

Then again, little in that conflagration was fair. It has become almost cliche to depict the loss of life as futile. In , Keneally breathes new life into the old lie by viewing it from the perspective of those "born colonial". The irony of fighting to defend a "motherland" you have not only never seen, but which either exiled your ancestors or invaded them, is not lost on Keneally's brave young soldiers. When the Australian, New Zealand and Indian forces first see the "dour, unwelcoming terraces" of "fabled England", he notes that "some of the young soldiers must have secretly asked themselves if this was what they had volunteered to die for".

The antipodean point of view also detects humour in this meaningless waste of human life, albeit of the blackest hue. Returning to Australia on leave, Naomi's ship suffers a suicide epidemic, after the otherwise seemingly sane Padre Harris throws himself overboard. As the ship edges closer to Australia, others copy the "fatal trend", unable to face life back home. "Within an hour orderlies armed with rifles had been placed on sentry duties on the decks. Perhaps - as Nettice said with dry humour - to shoot anyone who thought of killing themselves."

Keneally's most adventurous gambit could be the ending, which imagines two different outcomes to his story. It is a bold, and moving climax, one that remembers how fragile life was on the front. If the Germans, Turks, bullets and bombs didn't get you, there was always drowning, disease and even soldiers on your own side. For all that, Naomi and Sally - like their lovers and comrades in arms - were created by the conflict they volunteered for. When Naomi and the two rivals for her heart are shipped home to Australia, all three seek rapid returns to combat.

"It is stupid to think of this … as if it was a machine to make us true sisters. But that's the way it's happened," Naomi tells a pioneering female doctor, Airdrie, punning on "sister" to exemplify how their medical experiences have achieved what sibling relationship could not manage: true love and genuine understanding.

It is the supreme tragedy of this brilliant, humane novel that such intimacy is forged out of the profoundest suffering. "We're so accustomed to dreadful things now," Naomi says, at last, "that we might need to live together because no one else will understand the things we have seen."