April 28, 1942 was a terrible day for the Australian fighter pilots defending Port Moresby from Japan's relentless bombing campaign. Two were shot down and killed.

But the battle was most notable for how it ended.

As the Japanese bombers turned for home, they dropped four large mail bags with white streamers attached to them.

Inside the bags were 395 handwritten letters, carrying messages of love and hope from deep within Japan's war machine.

The authors were Australian soldiers stationed with 'Lark Force' at Rabaul, north-east of Port Moresby.

It was the most violent — and unusual — postal delivery in Australian history.

But by the time the letters finally reached their intended destinations, the men who wrote them would be dead.

'Don't worry, I'm OK'

On January 23, 1942 — just six weeks after Pearl Harbour — the Japanese arrived on Australia's doorstep when they attacked Rabaul.

Lark Force was under-resourced and under-prepared and could only offer token resistance.

The town was captured in just a few hours, and more than 800 members of Lark Force were taken prisoner by the conquering Japanese.

Nothing was heard from these men until the four mail bags dropped from the sky over Port Moresby three months later.

Authorities eventually decided that the letters would be delivered.

In his letter, Ronald Freeman tells his wife he has been "worrying about [her] all the time". ( Supplied: Vicki Freeman )

In September 1942, a postman walked up Matthews Street in the Sydney suburb of Punchbowl.

He slipped a letter into the mail box at number 55, oblivious to the mixture of joy and anxiety he'd just delivered to Dorothy Freeman.

The letter was from her husband Ronald Freeman, a gunner with the 17th anti-tank battery in Rabaul. It was full of reassurances and tenderness.

"My father was obviously very concerned about my mother … He knew my mother was a worrier and he was trying to say, 'Don't worry, I'm OK'. Obviously, he wasn't," explains Dorothy's daughter Vicki Freeman, who was just two at the time.

The letter takes pride of place in her collection of material about her father's wartime experiences. Sergeant Bob Burrowes was another Australian POW at Rabaul who wrote to his family. ( Supplied: James Burrowes )

At one point in the letter, Ronald asks: "How are you keeping anyway, it's getting closer, isn't it, to the new addition?"

When Ronald was captured, Dorothy was pregnant with the couple's second child. By the time the letter arrived, Vicki had a little sister named Gay.

Ronald signed off: "I love you, I love. Kiss Vicki for me. Your loving husband."

"I still do have tears in my eyes when I read the letter," Vicki says.

Another letter was delivered in the Melbourne suburb of Middle Park.

It was addressed to Mrs Alice Burrowes and was from her eldest son, Sergeant Bob Burrowes.

Jim Burrowes is 95 now, a former war veteran himself, and Bob's last surviving sibling.

He remembers the delivery of the letter caused some tension in the family. His sister Pat had intercepted it before her mother could see it.

"But it came out later, of course. She was trying to protect Mum, but I don't know what she could have done, of course. But that was my sister," Jim recalls.

Like Ronald, Bob also tried to sound optimistic.

In his letter, he asked his family to "keep the old bike in good nick as I will need it again".

Defiant humour

The Australian War Memorial holds several of the letters in its collection. Listen to the program The History Listen looks at the 'last letters' of Australian POWs and the worst maritime disaster in Australian history. Read more Read more

"What's very poignant for me is knowing that what I'm reading is exactly what this person experienced," says its assistant curator of private records, Tracey Langner.

Some letters strike a defiant note — like that of Sergeant Bob Hannah from Melbourne, who wrote: "Would you please see that the office and [my] friends are advised that I am still in the land of the living and intend to stay that way."

Others, like Gunner Les Lyons, a country boy from Junee in New South Wales, couldn't resist a bit of gallows humour.

"I have said more prayers than ever in my life — tell Marie those beads she bought me are nearly worn out," he wrote.

Somewhere in the South China Sea

After the letters arrived, nothing more was heard from the Lark Force POWs.

Unknown to the men's families, three months earlier, in late June 1942, they had been marched through Rabaul to the harbour.

Many were sick; all were malnourished.

At the docks, they had struggled up the gangway of a Japanese ship called the Montevideo Maru, then descended on ladders into its dark hold.

They were joined there by over 200 civilian captives.

All Australian POWs on board the Japanese ship, Montevideo Maru, were killed. ( Supplied: Australian War Memorial )

The men were being transported north to Hainan Island in southern China.

Although it was carrying prisoners, the Japanese did not mark the Montevideo Maru as a POW ship.

In the early hours of July 1 1942, as it steamed through the South China Sea, the Montevideo Maru was torpedoed by an American submarine called the USS Sturgeon.

All the POWs on board were killed.

They represented a cross-section of 1940s Australia.

There were public servants and plantation owners, missionaries and rugby internationals, members of the Communist Party and members of the Salvation Army.

Among the dead were the brother of former prime minister Earl Page, the uncle of former Labor leader Kim Beazley, and the grandfather of musician Peter Garrett.

A 'random act of kindness'

But one puzzling question remains: why did the notoriously brutal Imperial Japanese Army organise the letter drop?

Ms Langner says it was set in motion by a Japanese private after a random act of kindness by a group of Australian soldiers.

Kokichi Nishimura supervised a POW work detail in Rabaul which cleaned and repaired weapons. He learned from his Australian captives about an unusual act of wartime chivalry.

The day before Rabaul fell, the Australians had shot down a Japanese bomber, killing all its crew.

The dead Japanese airmen were given a proper and respectful burial by the Australians.

Nishimura determined to repay this act of humanity. He lobbied his superiors to undertake the air drop of letters — and amazingly, they agreed.

Memory and war

The sinking of the Montevideo Maru is the worst maritime disaster in Australian history.

Unlike Gallipoli and Kokoda, or Changi and the Thai-Burma Railway, it hasn't entered into Australia's national memory.

Yet it remains a seminal event in the individual histories of many Australian families.

For Jim, it's the loss of a big brother.

"I wish [he] was with me now. I'm so sad [he] suffered the fate of the Japanese during the war," he says.

When asked what she might say if she had a chance to write a letter to her father, Vicki says: "Dear Dad. I wish I'd known you. I think I missed out on having a father... Please come back."