An employer asks 22 construction workers to work a 32-hour shift without any rest.

He promises to pay a part of their wages - which have not been paid in six months - if they agree to the inhuman hours. They do so, but no money is forthcoming.

The boss then asks them to sign new contract papers and threatens them with repatriation with the help of repatriation agents if they dare complain to the Ministry of Manpower.

Under international definitions of human trafficking - where vulnerable people are forced or deceived into sex work or servitude - the employer would likely be charged with labour trafficking, says academic Sallie Yea, who reported the case to the authorities here after interviewing all the workers.

But in Singapore, the case may not be treated as trafficking, even if a Private Member's Bill on the Prevention of Human Trafficking is passed by Parliament.

Indeed, what constitutes labour trafficking is set to be one of the more difficult issues to be debated in the public consultation process for the Bill, which began last week.

More than 50 people, including academics, students, activists and business representatives, attended the first session. Three more sessions will be held over the next month.

The draft Bill has a section on offences related to "compulsory labour and slavery". It intends to criminalise acts such as the "selling, buying and hiring of persons for the purpose of labour trafficking". Consent of the victim will not be an impediment to enforcement.

A key task ahead is for the Bill to define and give examples of what kinds of acts could be construed as labour trafficking, say the groups working with migrant workers.

The International Labour Organisation has a comprehensive set of nearly 70 indicators on what could constitute labour trafficking. These were formulated for Europe and are grouped into three categories - strong, medium and weak.

Strong indicators of labour trafficking include excessive working days or hours, deception about the nature of the job, debt bondage and isolation, confinement and surveillance.

Debt bondage occurs when a migrant worker cannot leave an exploitative job because he has racked up big debts, usually to pay for recruitment fees.

"What we need is a clear set of such indicators on what will constitute labour trafficking offences in Singapore," said Dr Yea, an assistant professor from the National Institute of Education who has researched sex and labour trafficking in Singapore since 2009.

At the first public consultation on the Bill last Wednesday, participants tried to come up with their own set of indicators for labour trafficking. These included contract substitution or deceit about terms of contract, excessive work and work for little or no pay, withholding salary, threat of repatriation, unsafe working conditions, restrictions on freedom of movement and debt bondage.

Dr Yea told the group that one of the main problems in identifying trafficking cases is how many of such indicators a potential victim must fulfil in order to qualify as being trafficked. "That's a key challenge we must address," she said.

Addressing participants' concerns, MP Christopher de Souza, the man behind the Private Member's Bill, pointed out that there are several existing laws under the Penal Code and the Employment of Foreign Manpower Act (Efma) to penalise employers guilty of offences such as not paying or underpaying or incarcerating workers.

"Such acts are already illegal - and will remain so. But I have reservations about notching these up a level to trafficking," he said.

Activists such as Mr Jolovan Wham, executive director of Home, an anti-trafficking non-governmental organisation, counter that the Efma and Penal Code do not adequately address some fundamental elements of human trafficking such as deception and coercion, forced labour and the abuse of a victim's vulnerability.

"There is a big difference between individual acts of labour law violation - such as not paying a worker's salary - and human trafficking, which involves multiple, systematic forms of exploitation and abuse," he said.

He hopes the Singapore law will reflect the ILO definition of forced labour which is "all work or service exacted from any person under the menace of any penalty and for which a person has not offered himself voluntarily".

While sex trafficking has generally received more attention than labour trafficking, and is more accepted as a problem, Home estimates that instances of labour trafficking here may be far higher. "This is a problem that cannot be ignored, but one that will require both legal and cultural change in order to stop," said Mr Wham.

Whether offences against maids will be prosecuted under the new anti-trafficking law was another concern raised at Wednesday's session.

Mr de Souza said domestic workers would not be excluded "because we believe in equality before the law".

"However, we should look at the situation rather than the occupation to determine whether someone is trafficked or not," he said. "And only the most egregious cases will qualify."

The Bill aims to act as a deterrent against traffickers, he added. "But at the same time, it will help offer compassion to helpless victims; to respect their human dignity."

Meanwhile, experts have suggested other ways to help reduce chances of labour trafficking. One way is to allow foreign work permit holders greater flexibility to change jobs. "The reason many workers put up with exploitative conditions is that complaining will almost certainly make them lose their job," said Mr Wham.

Tackling the debt burden workers face is another way forward. Ambassador At Large Chan Heng Chee, Singapore's representative to the Asean Intergovernmental Commission on Human Rights, said that it would be good if employers - including those who hire domestic workers - could bear the recruitment fees so that foreign workers will not be in such heavy debt and in bondage to the recruitment agents.

"When they have to pay back debt, it is highly likely that a worker would stay in the same job even if he is mistreated," she said.

However she acknowledged that even if the suggestion is taken up, it will take time to be implemented as it involves higher costs for industry employers and for households.

But on its own, getting employers to bear the recruitment fees is not the panacea, she said. Workers should know what employers are doing on their behalf.

"It will produce better and loyal workers and, dare I say, higher productivity," she said.