The ministry added that the study didn't consider the "unique nature" of domestic work, saying that work and personal time "cannot be easily differentiated."

Privacy, freedom

If the survey is representative of Singapore as a whole, more than 140,000 maids in Singapore might be employed in exploitative conditions, the report said.

"We leave our families apart just to work hard and save money," one worker who participated in the study said. "But we just need some privacy (and) freedom, not for a week but for a day only.

At least 90% of the workers surveyed reported working excessive hours or days, while more than a third said they either had bad living conditions or low to no salary. Of those interviewed, 84% said they worked more than 12 hours a day and 41% said they were made to work on their single rest day.

Some said they were forced to massage their employer. A third said they were monitored by surveillance cameras.

Research Across Borders Director Anja Wessels told CNN she was "shocked" by the extent of the report's findings, saying they reflected only "the tip of the iceberg."

"These women mainly endure these situations out of economic stresses ... in one third of the cases in our study, the worker was the only bread winner in their family, which means if they do not work and send money back home, they will threaten the survival of their family," she said.

A large number of cities and countries across Asia and the Middle East have domestic helpers in their homes to assist with daily tasks, including Taiwan, Hong Kong, Dubai and Saudi Arabia.

In Asia, Singapore is second only to Hong Kong for having the largest number of documented foreign domestic workers employed in their country, typically young women from Indonesia and the Philippines.

Singapore's invisible workforce

Domestic workers make up 17% of Singapore's total workforce, according to the report, with an estimated one in three households relying on them for housekeeping and caring duties.

An estimated 56% of foreign domestic workers hired in Singapore are from the Philippines, while another 32% are from the Indonesia, the report said, although official statistics have not been released.

They mainly migrate to the city for economic reasons, to supplement their family's income back home. Remittances home from Filipinos and Indonesians working abroad in 2015 was more than $45 billion (S$62 billion), a sizable chunk of both countries' GDP, the report said.

But despite their huge contribution to Singapore, there are very few protections for Singapore's foreign domestic workers. Unlike Hong Kong, Singapore doesn't guarantee a minimum wage for maids and guidelines on working hours only call for a "reasonable workload."

However, a weekly rest day has been mandatory since 2013.

"They are often called the invisible workforce," Wessels said. "Singapore substantially relies on the migrant workforce, not only foreign domestic workers but also the hospitality workers and the migrant workers."

An employment contract is not mandatory when hiring them, there are few legal protections against paying unfair wages and there are only basic rules around what work they can and cannot do, the report claims.

"(The) results show the majority of workers endure labor exploitation out of economic distress, with no viable alternative but to remain in adverse working and living conditions," the report said.

'We are human, not robots'

One in four reported being a victim of violence, both psychological, through name-calling and verbal abuse, as well as physical -- hitting, spitting and pushing.

Wessels said she believed sexual violence was also being under reported. "It might be shame about the situation, there might be (job) insecurity ... or you might internally attribute what is happening to you, you might think it is your fault," she said.

According to the report's findings, the average monthly income of the workers interviewed was $381 (S$515) a month, often including meals and board, reduced to a net income of $158 (S$225) if they send money home to their families. Overall, the average monthly wage for an average Singaporean was $3,694 (S$4973) as of 2013, making a foreign domestic worker's salary just over one-tenth the average.

More than a third of domestic workers were also forced to pay for necessities, such as food and soap, despite government guidelines saying they were not supposed to be.

"Sunday is our off days, meanwhile Sunday still normal day, need to work first, tidy the house, walk the dog out, prepare their breakfast, clean toilet first ... then leave the house around 9 or 10 and must reach the house before 5 or 6 (at night)," said one worker interviewed by the report authors.

"After reach out must do house work again, wash car, cook dinner, same routine ... How you can consider that Sunday is off day? Please be good to us also. We are humans not robots," said another.