Up to 11,000 children may have been sold to European families, with both parties being given fake documents.
Some were reportedly born into "baby farms" that sold children to the West.
Sri Lanka's health minister told the Dutch current affairs programme Zembla he would set up a DNA database to help children find their birth mothers.
About 4,000 children are thought to be have ended up with families in the Netherlands, with others going to other European countries such as Sweden, Denmark, Germany and the UK.
One adoptee called Rowan van Veelen, told the BBC earlier this year that he had travelled back to Sri Lanka to try to find his birth mother 27 years on.
He was part of a Netherlands-based social media network that tried to match Sri Lankan birth mothers to their estranged adopted children.
"The adopted children and the mothers got the wrong information, which makes it really hard," he explained.
The Facebook page dedicated to reuniting Sri Lankan adoptees with their parents has hundreds of members
"We want to make a DNA bank with all adopted children from [the] Netherlands who can search if we have siblings there.
"Then we could ask other countries like Sweden, Denmark and Germany to give their DNA also in the bank too."
Adoption documents 'falsified'
The Dutch filmmakers from Zembla started looking into the allegations after the Dutch Council for the Administration of Criminal Justice and Protection of Juveniles advised the government in November 2016 to consider banning foreign adoptions because of unethical practices in some of the children's origin countries.
Norbert Reinjens, a researcher for Zembla, told the BBC that they had found evidence that all kind of documents were falsified by adoption authorities - including birth certificates, the names of children and the identity of biological parents.
"In some institutions there were 'acting mothers' who were paid to pretend to be the biological parents while handing them over," he said.
People now aim to match the correct information and records with estranged children in Europe
In the documentary locals allege that some hospital workers worked alongside the networks.
Some new mothers at a hospital in Matugama, western Sri Lanka, were reportedly told their children had died, when they were actually sold abroad for adoption.
One woman told the documentary makers she was paid 2,000 rupees (£23; $30) by someone connected to the hospital to act as a baby's mother.
Looking for answers
Azzam Ameen, BBC Sinhala, Negombo
Many mothers had come to the western coastal city of Negombo from all over Sri Lanka in search for answers. They had heard of a visiting group trying to find their missing children. They had barely any documents. One mother had a picture of her child taken in 1989, but had no other information on the child's adoptive family or location.
One mother told me she checked the adoption papers of her daughter and found all the information to be false. It seemed that a "fake mother" had been produced in court for the adoption.
Another woman confessed to have accepted money to claim she was the mother of a girl. She said she did it as she was desperate for cash.
Only mothers had showed up, not fathers. Most of the women said they had to give up their babies for adoption because their husbands had abandoned them in the first place.
In Sri Lanka, we often hear about happy reunions of Dutch adoptees and Sri Lankan parents in newspapers. But after talking to these mothers it became clear there could have been a massive scandal in the '80s and '90s.
Sri Lanka temporarily banned intra-country adoptions in 1987 when one "baby farm" was raided, and 20 newborns were found inside.
Reports from the time said the women there were being held in "prisonlike conditions", surrounded by a 10 ft (3m) wall.
Sri Lankan Health Minister Rajitha Senaratne denied the government held any responsibility or knowledge of the networks.
But in an interview with the documentary makers he said he would look into setting up a special office to try to match the children involved.
"It is very wrong, it is against human rights of those families - so that has to be investigated. I myself will take the responsibility for that investigation."
One mother, Renuka Abeysinghe, told the BBC that her daughter was taken in 1992.
Mothers interviewed by the BBC said they had no knowledge of their children's whereabouts
"I will be very happy if I meet her again. I gave her for adoption because we were poor and we didn't have a way to live.
"They told me they took her to Germany but we had no information after that."
Another, Ms Sirimavathi said she was alone when she gave up her daughter.
"I don't have parents, and all my close relatives were abroad when I got my child. I didn't have anyone. I didn't have a place to stay, " she said.
Ms Sirimavathi said she had "no one" when she gave up her child
"I cried a lot when giving away the baby, still I am crying.
"My wish is to see her once, I don't want anything from her."
The Dutch State Secretary for Security and Justice confirmed to Zembla he was looking into the allegations.
Additional reporting from Azzam Ameen, BBC Sinhala