New Zealand has an estimated 600 people living in some sort of slavery, according to a report by campaign group Walk Free Foundation.
The anti-slavery foundation released its Global Slavery Index which said nearly 36 million people are enslaved worldwide, two-thirds of them in the Asia Pacific region.
The report defined slaves as people subject to forced labour, debt bondage, sexual exploitation, and forced marriage.
Mauritania and Uzbekistan had the highest proportion of its population in modern slavery, at 4 percent and 3.97 percent respectively. It was followed by Haiti, Qatar and India.
In terms of absolute numbers, India had the most with 14.2 million people, followed by China with 3.24 million.
New Zealand ranked fourth best of the 167 countries surveyed but experts warned there was no room for complacency.
Stephanie Lambert, executive director of Justice Acts which combats trafficking and slavery in New Zealand, said any number was too high when it came to slavery.
"Every number is a person's life that has been exploited and abused and we don't want even one slave in New Zealand."
Ms Lambert said experts and the government needed more information about modern slavery in New Zealand to fully tackle the problem, as there was a lack of data.
Fiona David, a spokesperson for the Walk Free Foundation, said the cases in New Zealand included two men who were charged earlier this year with arranging by deception the entry of 18 Indian nationals in New Zealand. The men have denied the charges.
There have been cases of the exploitation of migrant workers, including Filipino construction workers in Christchurch who worked at weekends without pay, or were not paid at all for several weeks.
Last week, the Employment Relations Authority ordered a Vietnamese restaurant, Little Saigon, to pay an immigrant chef $175,000 for unpaid wages incurred over five years and other penalties. The ruling said Vu Nguyen and his brother, Bao, lived in the owner's garage, worked 66 hours a week without pay and ate in the restaurant.
And yesterday, a director of a Wellington immigration agency received 10 months home detention, 300 hours community work and ordered to pay $30,000 reparation for exploiting six Chinese chefs.
The government has already changed labour laws to apply to foreign fishing vessels, and planned to introduce tougher penalties for employers who exploit migrant workers, among other changes.
But Craig Tuck, the founder of Slave Free Seas, which looks into modern-day slavery, said the country could not rely on regulation alone.
"You can have all the laws that you like, but if people aren't going to act on them, if they have no teeth or they can't understand how to implement them, then it's a waste of time," he said.
Mr Tuck said businesses intent on exploiting others had always worked around the law, and the government had to make sure it stayed a step ahead.
The Walk Free Foundation report placed New Zealand second in the region when it came to government action on slavery, but the country got five out of 12 for central government coordination and accountability.
Organiser of the Union Network of Migrants, Dennis Maga, agreed with the report and said labour inspectors had been hesitant to handle some cases.
He said inspectors should randomly check on employers who had exploited migrant workers and pursue more cases, to show workers that they were being protected.