"Clearly it's an indication that authorities are not fulfilling their duty to combat trafficking," Matthew Smith, the group's executive director, said in an interview.
Reuters reporters joined the volunteers - mainly fishermen and other villagers - on one of their near-daily patrols for boat people who might be hiding along the coast.
The group has limited funds and poor equipment compared to the wealthy and well-oiled smuggling networks, said Jessada Thattan, another volunteer.
"Mostly we are slower than them. They use better boats than us," said Jessada.
The volunteers are partly funded by Takua Pa district, but some costs, particularly gasoline, come from their own pockets.
Volunteers said they had yet to catch any smugglers or traffickers. But they have discovered more than 220 Rohingya Muslims and Bangladeshis over the past three months and handed them over to immigration police.
Additionally, more than 130 suspected trafficking victims, mostly Bangladeshis, were found dumped by traffickers in a remote coastal area of Phang Nga. Many of them had been abducted or tricked onto prison ships in the Bay of Bengal where conditions resembled the horrors of the trans-Atlantic slave trade, Reuters reported in October.
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One group was sick, half-starved and dehydrated by the time volunteers found them.
Smugglers, the volunteers said, once hid groups of 300 to 500 Rohingya on the islands that dot the district, but have now grown more cautious.
"The smugglers' tactics are changing," said Jessada. "Before we would find groups of 100 together on an island. Now, the smugglers break them up in to groups of 10 or 20 to make it easier to hide and transport them."
Local people were slowly coming on board to help establish a network of anti-smuggling informants, said Jessada.
"Almost 100 percent of the fishermen in the area are on our side now," he said. "They let us know if there are suspicious activities on the islands or if someone is making a large delivery of food to a remote area."
Despite the enormity of the task, the volunteers said they were being spurred on by the disconnect between government policy and practice.
"The government can announce as many anti-trafficking policies it wants," said Cherdchai Papattamayutanon, a village chief in Takua Pa who helped form the group. "The truth is, we're at the frontline here and we're alone."