"My body aches. I cannot do it," she says.

To earn 65 cents an hour, Vien is supposed to sew 50 garments every hour - but she can manage only about 30.

She is one of thousands of under-age children working in Cambodia's more than 700 factories which employ an estimated 700,000 mostly young Cambodian women.

Across Asia, millions of poor and desperate garment and footwear workers are toiling for long hours up to seven days a week for as little as $100 a month.

Now 16, Vien stopped going to school in rural Cambodia six months ago, when she was only 15, and borrowed the identity card of a relative to claim she was 18 to get a job in a garment factory on the outskirts of the teeming Cambodian capital.

Under Cambodian law, garment factories cannot employ children below the age of 18 to work in what unionists and activists say are sweatshop conditions, although they can employ children aged 15 or above for light duties for up to eight hours a day.

Vien says she has no other employment opportunities in a country where many young women are forced into a flourishing sex trade.

"It's difficult work and I only save ... [$26] a month to send home to my family," she says.

But Vien says her parents need to buy a house and pay medical costs and her 14-year-old brother and 12-year-old sister need money to go to school.

Buyers of "made in Cambodia" garments around the world have reason to cringe about the conditions forced on Cambodian clothing makers in an industry that generates $7.8 billion each year and provides 80 per cent of the country's export earnings.

Unions and activists say employing children to work alongside adults with equal pressure to meet targets and work overtime is common in factories, especially hundreds of shadowy unregulated and unlicensed factories backed by politically-connected figures.

Human Rights Watch has documented the use of child labour in violation of local and international laws in at least 11 factories, including workers as young as 12.

According to the March report, one woman at a factory supplying H&M clothes "estimated that 20 of the 60 workers are children".

Others told Human Rights Watch that "children work as hard as adults" and would stitch long hours into the night.

The New York-based organisation has also documented serious labour rights abuses, including forced overtime, pregnancy-based discrimination and anti-union practices that fail to protect workers producing brands such as adidas, Armani, Gap, H&M, Joe Fresh and Marks & Spencer.

About 90 per cent of Cambodia's workers are women, and if they get pregnant they are often sacked. One worker said women often wear tight clothes to hide their pregnancies.

Even going to the toilet can cause the wrath of factory bosses. Announcements like "don't go to the toilet … you need to sew" are made over loudspeakers.

Workers complain that temperatures are so high in factories they are forced to wear layers of clothes so their sweat will cool them. "If we don't wear many clothes, the heat burns our skin and pieces of fabric get caught in our throats if we don't cover our faces with masks," says Kong Savorn, 39, who works in a factory sewing Armani jeans.

In May, 18 of 39 garment workers packed into a minivan were killed when the vehicle collided with a bus in the country's Svay Rieng province.

Union leaders and activists in Phnom Penh told Fairfax Media that while Cambodia is portrayed as a model for the global garment industry, discriminatory and exploitative labour conditions, the imposition of short-term contracts that make it easier to control workers, poor government labour inspections and enforcement and aggressive tactics against unions prevent workers asserting their rights.

There have been episodes of workers fainting en masse in factories, which unions blame largely on the improper storage of chemicals used in garment production.

In November last year - more than eight months after Cambodian security forces opened fire on workers to crush industry-wide protests - the government of Prime Minister Hun Sen set a minimum monthly wage of $167, an amount unions say is not enough to live on.

David Welsh, country director in Cambodia of Solidarity Centre, a Washington-based international worker rights group, says no worker can survive on the minimum wage without working illegal overtime hours, effectively meaning long hours, six days a week.

"The minimum wage should provide a degree of basic needs and comfort," he says.

Oem Mom, 33, sleeps on a wooden bench with four others in a tiny room made of scrap timber and cardboard off a dank alleyway near the Phnom Penh factory where she works sewing designer jeans.

"Life is very hard … the wage is not enough to live on. I have to keep borrowing from loan sharks," she says.

Oem wants to work more overtime so she can send money each month to her grandmother who cares for her 10-year-old daughter and six-year-old son. "There is no overtime at the moment and my situation is going backwards," she says.

After November's wage rise, businessmen renting rooms to workers raised the rents, wiping out any improvement in the workers' earnings.

Oem says that during heavy rain her room floods with putrid drain water but when she and others complain the owner blames them.

There is no toilet or running water, like many of the rooms in the shanty towns that surround Cambodia's factories.

"If I had money I would move somewhere else," she says.

Cambodia's garment industry, dominated by foreign investments from Hong Kong, China, Singapore, Malaysia and South Korea, insists that attendance bonuses and housing and transport allowances make Cambodia's garment workers some of the best paid in the region.

The Garment Manufacturers' Association of Cambodia, representing 600 factories, says Cambodia sets the standards for other countries with an industry that accounts for 15 per cent of the economy.

The government insists its monitoring of the industry is transparent and competent, dismissing claims of systematic labour law violations and corruption as "groundless".

But Ath Thorn, president of the 100,000-member Cambodian Labour Federation, says many factories force workers to work Sundays, on weekdays until 9pm and sometimes overnight until 6am – all without overtime and with workers in fear of losing their jobs if they do not comply.

He says union representatives and workers who complain face dismissal and threats of legal action in politicised courts.

In mid-May, workers striking over the dismissal of a union representative at a Phnom Penh factory were attacked by up to 50 thugs.

Pav Sina, president of the Collective Union Movement of Workers, says many factories insist on workers signing only three-month contacts which are not renewed for troublemakers.

He says under-age workers use false identification cards because the workers are desperate to find work.

"How can we go against what they want?" he says.

The Solidarity Centre's Welsh says while the world focused on Bangladesh's garment industry after the collapse of the Rana Plaza building in 2013, which killed 1134 people and left thousands more injured, Cambodia's industry is structured in a similar way, catering to the same international markets.

"Cambodia has been painted as a model for the global industry – the claim is that factories are properly monitored for abuses and conditions," he says.

"But that myth was destroyed in early 2014 when Cambodia security forces shot more than 60 protesting workers, jailed prominent union leaders and sacked hundreds of workers," he says.

Welsh says many of 200 apparel brands that source garments from Cambodia became "skittish" following the turmoil, making the industry vulnerable to companies turning to other cheap labour markets such as Myanmar.

Myanmar now employs more than 200,000 workers, a tenfold increase since 2010, with an average of two new factories opening every week.

A garment worker in Yangon is typically paid an average $US80 $100 a month, including overtime. There is no minimum wage.

According to research by the ANZ, cheap labour in Cambodia, Laos and Myanmar – the so-called Mekong frontier – will help drive a manufacturing boom in south-east Asia in the next decade.

Welsh says that brands claim on their websites they are concerned about workers' wages and conditions in overseas factories but they actually set the conditions.

"Brands chase jurisdictions where there is extreme poverty, the rule of law is weak and it is easy for them to skirt labour laws," he says.

"The only way to stop that is for there to be basic global standards that apply in every country."

Among the brands commissioning garments in Cambodia only four have fulltime staff monitoring factories for labour abuses in the country.

Australian retailers that source from Cambodia - including Coles, Kmart, Target, Big W and Pacific Brands - say they comply with international standards and the number of garments supplied to Australian brands remains small, compared with other supplier countries such as China and Bangladesh.

But in April, the Australian Fashion Report by Baptist World Aid found that nine out of 10 companies supplying Australian consumers do not know where their cotton is sourced and most fail to pay overseas workers enough to meet their basic needs.

The deaths of 72 people in fire that gutted a footwear factory in the Philippines in May exposed abusive conditions in that country, where roughly one quarter of the 100 million people population live in poverty.

"The deaths should serve as a wake-up call for businessmen to stop these abuses … they should give their workers dignity," said Rosalinda Baldoz, the country's Labour and Employment Secretary.

Vien Dyna says she risks angering her factory supervisors by speaking out about conditions. "I'm worried I will get called in and have my fingerprint taken – that is a warning," she says. "If you get three warnings you are dismissed."

But Vien says she wants to help improve conditions for 14 other young women workers she shares a house with in a Phnom Penh suburb.

"We are together," she says.