Drift past an apartment building called Monterey on Queens Road, South Yarra, right across from the Albert Park golf course, and you might, if you were to notice it at all, register it is a fine example of art deco.
Nothing indicates that Monterey was Australia's own Bletchley Park, the British code-breakers' headquarters during World War II.
Those within Monterey were the cream of Australian, American and British code-breakers, code-readers and a small army of young women specialising in processing and analysing intelligence plucked from the air.
The secret work that took place in Monterey – and complementary intelligence slog in a suburban house named Nyrambla in Ascot, Brisbane – changed the course of the war in the Pacific.
Albury woman Joan Duff spoke nothing about what she did in Melbourne's Monterey for more than 60 years. When World War II was done she tucked away her secrets, travelled Europe, married a bomber pilot named Neil Fairbridge who'd been awarded a Distinguished Flying Cross, worked as a journalist and had two sons.
A few years ago, when she was aged 91, a small parcel turned up in Joan Fairbridge's mail.
In it was a badge of pure gold, struck by the Queen's medallist, and a note saying, "The Government wishes to express its deepest gratitude for the vital service you performed during World War II".
The parcel arrived on Australia Day, 2011, but it wasn't from the Australian government. The medallion was the Bletchley Park Commemorative Badge. The letter was signed by David Cameron, then prime minister of Britain.
It was for "service in support" of Bletchley Park, the British Government Code and Cypher School that famously cracked Hitler's "unbreakable" Enigma cipher system – the subject of a 2014 Oscar-nominated film, The Imitation Game.
Joan Duff's work took place in Australia, and the codes she and her colleagues dealt with were those of Japan's Imperial forces. But no such gesture of thanks has come from the Australian authorities to those numerous souls who were part of the ultra-secret operation that outsmarted the Japanese in the Pacific War.
I worked with Joan Fairbridge (nee Duff) in the late 1970s at The Border Mail, and never had a clue about the secrets she harboured. We've talked a lot since her Bletchley badge arrived.
Joan, who turned 97 a couple of months ago, believes she is one of only two women still alive who worked covertly on "sigint" (signals intelligence) in Melbourne.
The story of how a sophisticated signals intelligence network was built in Australia from scratch to track Japanese ships, planes, troops and battle plans is detailed in an absorbing new book by Canberra-based writer, David Dufty.
The Secret Code-Breakers of Central Bureau: How Australia's signals intelligence network helped win the Pacific War tells a series of rattling good tales of the people involved in this silent, mighty effort, including parts of Joan Duff's story. She was, after all, there from the start.
Aged just 20, she was chosen to work alongside the head of Navy signals intelligence, Lieutenant-Commander Jack Newman, as he built his secret directorate.
Raised in Brighton, with three brothers away in the military, Joan fronted the recruitment station next to Melbourne Town Hall, declaring she wanted to join the Army. The recruitment officer studied her school reports, gave her a note marked "secret" and sent her to Victoria Barracks.
There she met Lt-Cdr Newman. She confessed she had no special skills, not even touch typing, though thanks to the Presentation nuns who had taught her at Star of the Sea, Gardenvale, she could speak passable French. For some reason that still evades her, the powerful Newman asked her to start work that day.
"He then personally instructed me in the first of many tedious, monotonous, and to me, boring tasks in the field of signals intelligence," she says.
It was August 1941, just four months before the Japanese attack on Pearl Harbour.
"I was in traffic analysis: who was talking to who – Japanese ships, submarines and shore bases," she says. "What they were talking about … I had no idea.
"No one ever had the big picture. We weren't allowed to talk about it or even to associate with each other when we left the building."
Needing space, everyone moved from Victoria Barracks to the nearby Monterey flats.
Joan remembers the first computer: an IBM dubbed "Ivy" that took up an entire room.
And yet, "I had no idea at the time of the importance of the work that was being done by quite brilliant people with whom it was my privilege to serve", she says.
No one was even told when intelligence from Monterey meant a Japanese warship had been sunk.
Still, Joan got a wake-up at a tense time when Japan appeared to be winning, and even those at high levels feared an invasion.
Cdr Newman told Joan he hoped those in uniform would be made prisoners of war, but feared for her because she was a civilian.
" 'You could be shot', he told me," she recalls. "'As a spy?' I said flippantly."
The commander reminded her that she was engaged in espionage.
"I was radioactive. I'd been let in on too many secrets," she says.
"After the war finished, I wasn't allowed to tell anyone I'd worked in espionage, but I didn't want to talk about it anyway. People don't understand – we all wanted it behind us."
And yet, says Joan Fairbridge/Duff, she bears no regrets.
At 97, she has two sons, two grandsons and two granddaughters, of whom she is enormously proud. She regularly meets a group of friends from Albury-Wodonga who speak French over lunch at La Maison, and has a world of memories about which, finally, she is free to speak.
- The Secret Code-Breakers of Central Bureau: How Australia's signals intelligence network helped win the Pacific War. By David Dufty. Scribe ($49.99)
- This story first appeared on The Age