When Wang Ming Ming was five years old there was a knock at the door. A representative from the family planning unit in the Chinese town where she lived had an envelope to deliver to her parents.

Inside was a fine for 2,500 RMB. For Ming Ming's parents the sum, around $500, was equivalent to more than two years of her father's salary.

The family's crime? Ming Ming was their second child, a so-called "black child", conceived and born illegally during the Chinese government's one-child policy.

Ming Ming's family spent all their savings to afford the fine, but even with the money paid off, Ming Ming had to go into hiding.

The family planning unit in her city had made its own mistakes and had failed to prevent too many second pregnancies. More than its allowed quota of second children had been born.

Ming Ming had to go into hiding for another year to make up the following year's quota of forbidden siblings.

And so, until she was six years old, Ming Ming remembers running to hide whenever government officials turned up to check how many children were living in the house.

She thought it was a game and happily complied.

Soon after her sixth birthday, Ming Ming was allowed to start school and with the fine paid off, the family lived openly at last with their two children.

One of the harshest family planning policies in the world

A generation of Chinese citizens have grown up as only children. ( Reuters: Kim Kyung-Hoon, file )

An entire generation has grown up during the one-child era. Introduced in 1979 and only overturned in 2015, the policy was an attempt to slow China's rapid population growth.

But policing the bedrooms of a nation was difficult. To achieve it, China introduced one of the harshest family planning policies in the world: women were frequently traumatised by forced sterilisation and abortion, scans to check that government-issued IUDs were in place, or regular blood tests to prove they were not pregnant.

If those strategies were not enough to deter couples from trying for a second child, then the threat of losing a job or social status was a common threat.

For government workers, a second child meant almost certain demotion, a drastic salary reduction or the sack.

There were exceptions though.

Farming families or women from China's ethnic-minority populations, such as Tibetans and Uyghurs, were exempt if their first child was a girl.

This in part acknowledged that these traditional families are more likely to see sons as essential to continuing rural life.

A surprisingly common secret

In 2015, the policy was abandoned and all Chinese couples are now officially allowed to have two children.

Karoline Kan, in her mother's arms, was a second child born during China's one-child era. ( Supplied )

But when you get to know Chinese people it is surprising how many have stories just like Ming Ming.

Karoline Kan was born in 1989 after her mother defied the one-child policy to give birth to her.

As a substitute teacher, Kan's mother knew that she was not supposed to break the law. But despite having a son already in a society that preferences male children, she desperately wanted a second baby.

Kan's mother only revealed her pregnancy to her husband and close family members.

"My grandparents could, by no means, understand why my mother wanted a second child — she already had a son," says Kan, who wrote a book about her family.

Negotiating bureaucracy

What is the one-child policy? Majority of couples allowed only one child

Majority of couples allowed only one child Policy developed in face of food shortages and high birthrates

Policy developed in face of food shortages and high birthrates Strictly enforces with fines, inspections and even forced abortions Read more here

The longing for more than one child was often most deeply felt by women.

"My father didn't want a second child but it was important for my Mum," says Susan, who was born in Yuncheng county in southern Shanxi province in 1982.

"She was more traditional in thinking and really wanted a son."

At the age of four, Susan became a big sister.

"My mother decided to go to the countryside to give birth," she says. "I guess she felt [she would be less likely to raise suspicion]. She was worried about the official registration of her second child."

At the time, it was common for couples to register their second child under the name of a relative who had no children.

That way, fines could be avoided and a "hukou" — an official registration document that allows the person access to education and medical care — could be applied for.

Susan's mother gave birth to her longed-for son and a month later her father went to register the baby. Without a relative to use as cover, Susan's parents were fearful of the penalty they may face.

To her father's relief, he was told that the baby could be registered without receiving a fine.

But there was a different problem.

In 1953, China had introduced a system of planned purchase and supply to monopolise the selling and buying of grain and oil.

Every Chinese family received grain ration coupons from the government that were distributed according to the age and number of family members. A second child was not included.

As he grew, Susan's family had to pay steep market prices to buy additional grain for their son. It was a major financial sacrifice for the family.

But even so, Susan's family felt lucky.

Susan's family still does not understand why authorities overlooked the birth of the little boy and did not issue a fine, like in Ming Ming's case.

Perhaps they felt sorry for the family — some people in China still consider a daughter to be bad fortune compared with a son which has led to other social problems.

Luck of location, or not

But their luck could also be attributed to the province they lived in.

The one-child policy was strictly enforced in eastern and north-eastern China, but local governments in middle provinces such as Shaanxi and Shanxi were often more flexible.

Perhaps because of this flexibility, it was surprisingly common for families in Susan's town to have more than one child.

"I clearly remember one day when I was in sixth grade our teacher asked the class of 60 pupils how many of us had a younger sibling," she says.

"Over half the class raised their hands."

For most families, it was "just" a matter of paying the fine.

"The parents of one of my classmates nicknamed their son Zhang Jin Jin (Zhang gold gold) because he was so expensive," says Susan.

Pregnant and in hiding

Many Chinese families found ways to work around the one-child policy. ( Supplied: Sydney Film Festival )

But for many couples, attempting to have a second child led to life-long trauma.

Zhang Ting grew up in the remote north-western province of Gansu.

When her cousin was 10 years old, Zhang's aunt and uncle decided to try for another baby.

"From the first day of my aunt's pregnancy, she and my uncle had to hide in order to avoid getting caught by the officials from the local family planning unit," says Zhang.

Eight months into the pregnancy, the couple went to stay with another aunt to hide before the birth. Someone reported them.

"One night a dozen people arrived at their hiding place," Zhang says. "Among them was a doctor from the local hospital. He gave my aunt an injection to induce her and two days later, she delivered a stillborn baby."

Zhang's aunt, now 46, has missed her chance to try for a second child now that the policy has changed, but she mourns for the chance she lost and the brutal way her second child was taken from her.

The barbaric treatment that Zhang's aunt received was not uncommon.

Apart from forced abortions, family planning authorities kept details of the menstrual cycles of women of childbearing age — pelvic examination results and blood tests. Most women were fitted with IUDs and summonsed for mandatory tests every three months.

But despite witnessing the brutality of these policies up close, Zhang's desire for more than one child was stronger than the fear of facing what her aunt had gone through.

She managed to give birth to three children, all born during the one-child era.

Zhang paid a $2,000 fine for having more than one child. ( Reuters )

"The family planning unit in my hometown in Gansu demanded that I go every second month to do a blood test to prove that I wasn't pregnant," Zhang says.

"After I moved to Beijing for work, I had to send proof from there, again every second month."

Women would often cheat and have a non-pregnant friend or relative do the test, Zhang says.

Zhang's two sons were born in 1998 and 2010 and her husband paid the $2,000 fine for the birth of their second child.

But two years later when Zhang gave birth to a daughter, the fine had gone up to $4,000.

Zhang negotiated with the officials — offering gifts of alcohol and cigarettes in return for a discount on the fine to $2,000.

Some see benefits to the policy

Not everyone in China condemns the one-child policy.

Some praise the policy for having lifted large parts of China out of poverty, and the government estimates that it has prevented 400 million births and contributed to China's dramatic economic take-off since the 1980s.

It was a lucrative financial resource for the government too, with estimates suggesting that over the 30 years of the policy, billions of dollars were collected in fines from families who had an extra child.

But for families like Susan's, Ming Ming's and the many others who defied the one-child policy, that forbidden sibling was always worth the risk.

*Residual fear of being discovered remains strong and most of those interviewed for this story declined to be photographed.