The clan offered their only available

In the corner, a pair of dinosaurs print keychains. And if that weren't strange enough, the keychains are graphs.

In eight cities, analysts, developers and interested others gathered to vie for glory and prize money in the nation's largest ''hackathon''.

Paul Taylor from science education provider Quantum Victoria and his partner Kim Tinson, who works in childcare, used a 3D printer to create keychains and rings graphing asylum seeker boat arrivals to Australia since 1976. ''We decided we wanted to make data fun, meaningful and engaging to the general public. We want people to enjoy data,'' Mr Taylor said.

Previously the event was held only in Canberra and Sydney, but this year it expanded to Melbourne, Adelaide, Perth, Brisbane, the Gold Coast and Tasmania.

About 150 people attended the Melbourne event, which was held at Fairfax Media's Melbourne headquarters.

Data analyst Andrew Grealy and his team devised a tool to download NSW crime data going back 17 years and turn it into an interactive dashboard allowing people to track offences by area.

He said the biggest issue his team faced was the way the data was presented, as they had to spend a lot of time converting the information into a form that they could analyse.

Other projects at GovHack Melbourne included visualisations of census data, an app for discovering the native flora and fauna in an area, and a tool for translating government data websites into other languages.

Organiser Pia Waugh said about 150 teams from across the country had entered GovHack in the hope of competing for a slice of $170,000 in prizes.

''GovHack is all about trying to bring together government people along with industry, academia and developers - the fairly awesome technical community we have in Australia - to do some pretty cool stuff,'' she said. ''It's about getting better transparency,'' she said.

As a child in rural Bangladesh, Orola Dalbot, 30, enjoyed growing up around her stepfather, Noten. Her father died when she was small, and her mother remarried soon after. Noten was handsome and energetic, with curly dark hair and a broad smile. "I thought my mother was lucky," Orola says when we meet in the dusty, sun-baked courtyard of her family home in the central forest region of Modhupur. "I hoped I'd find a husband like him one day." When she reached puberty, however, Orola learned the truth she least expected: she was already Noten's wife.

Her wedding had taken place when she was three years old in a joint ceremony with her mother. Following tradition in the matrilineal Mandi tribe, an ethnic group of about two million people spread across hill regions of Bangladesh and India, mother and daughter had married the same man. "I wanted to escape when I found out," says Orola. "I was shaking with disbelief."

Disbelief was more or less my reaction a few days earlier when, by chance, I'd first heard about this marriage custom. I was visiting the remote Modhupur region to report a story about Mandi women fighting deforestation. My travelling companion was an eminent Bangladeshi environmentalist called Philip Gain, who had been studying the area for more than 20 years. As we drove through the khaki- coloured hills, we talked generally about how Mandi women were the property-owning heads of their households. Gain, 50, a professorial man in a suit jacket and tie who runs the Dhaka-based activist organisation Society for Environment and Human Development (SEHD), told me how they shared power with men and had far more independence than women in the majority Bengali population.

Then Gain mentioned the mother-daughter joint marriages. He explained that among the Mandi, widows who wish to remarry must choose a man from the same clan as their dead husband to preserve the clan alliance. The only available single men, however, are often much younger men in their late teens. So the custom evolved: a widow would offer one of her daughters as a second bride to take over her marital duties – including sex and child-bearing – when the girl came of age. "It's not common these days," said Gain. "But it still exists among a few Mandi families."

Bangladesh is a deltaic country where most of the 160 million people are Bengali Muslims. It is better known for its flood plains and typhoon-lashed coasts, but its southeastern and central hills are home to ethnic minorities who mainly practised animism until Catholic missionaries arrived in the late 19th century. The Mandi, who number 25,000 in the Modhupur region, live a six-hour drive and a world away from the frenetic capital Dhaka.

Orola is cooking rice and lentils for breakfast on an open fire when we arrive at her hamlet, a cluster of mud houses flanked by scrubby fields. Her family members are all there: her 51-year-old mother Mittamoni, her stepfather and husband Noten, 42, her maternal grandmother and an assortment of children ranging from babies to teenagers, fathered by Noten with both his wives. Everyone is doing household chores in the weak morning sunlight.

The family's marital arrangement is an open secret in this small Modhupur community, but nobody, Orola says, ever mentions it. "For years I wanted to talk to someone about it because I was lonely. But people think it's un-Christian, so they ignore it." Missionaries have converted the majority of the tribe's local population. Traditional rituals, such as sacrificing goats to restore a sick person's health, are frowned on by the clergy and have waned. "Bridegroom kidnapping", another rare custom in which Mandi women abducted potential suitors and held them hostage until their wedding day, has also died out. A handful of mother-daughter joint marriages have most likely survived because, like most unions worldwide involving multiple spouses, they serve an economic purpose.

"My mother couldn't manage her land and household by herself when my father died of fever," explains Orola. "She was still in her mid-20s, so she was entitled to claim a new husband as a replacement from my father's clan." The clan offered their only available bachelor at the time, Noten, who was then aged 17, on the condition he marry Orola, too. Since Mandi marriages represent the consolidation of wealth between two clans, the second, younger wife is a trade to ensure the birth of more children to add to the family's overall wealth and power.

"I was too young to remember the wedding. I didn't know it had taken place," Orola tells me while she stirs her pots. Although such an arrangement is not considered incest or even child abuse in Mandi culture, where early marriage is the norm, she was distraught to discover she was forced to share her mother's husband. "The last thing I wanted was to be married to Noten. I wanted a husband of my own."

The situation was doubly unjust for her, she says, because ethnic Mandi women traditionally choose their own partners. It is women who make the first romantic move, and also propose marriage. Property is passed down the female line, and men live in their wife's household when they marry. She watched her female friends embark merrily on their love lives and felt so isolated that she considered suicide. "I felt trapped, like an animal."