Andrew Drummond, who has specialized in exposing the activities of foreign criminals and scams targeting foreigners, expressed dismay at the growing use of Thailand's criminal defamation laws and Computer Crimes Act to intimidate and silence people who expose wrongdoing.

"I have enjoyed my time in Thailand where I have made many Thai and foreign friends but there comes a time [when] having too much knowledge, which I cannot keep to myself, becomes too dangerous," he said.

"My well being has been threatened as have those of my three children. This is not of course the first time but the recent threat came from a group of people who have killed with impunity before and have even had police set up people on false changes."

British-born Mr Drummond, who has worked in Australia and filed for newspapers including Fairfax Media and the Australian, lashed out at what he called the "harassment" of Australian journalist Alan Morison and his Thai colleague Chutima Sudasathian who are being sued by the Royal Thai Navy for re-publishing in their Phuketwan news outlet a 41-word paragraph from a Reuters report on the mass exodus of Rohingya Muslims from Myanmar.

Phuketwan has been campaigning since 2008 for the rights of Rohingya who have been described by the United Nations as among the world's most persecuted people.

Mr Morison, a former senior editor at The Age, and Ms Chutima face fines or years in jail if convicted at a trial set for July.

Mr Drummond also condemned defamation and Computer Crimes Act proceedings against British-born Andy Hall who represents the right of Myanmar migrant workers in Thailand.

"The harassment of Andy, Alan and Chutima has no place in the country calling itself free. Thailand's inability to take criticism, and its greatly flawed justice system, are major handicaps to its progress," he said.

Mr Drummond has defeated numerous defamation actions brought against him by foreigners in Thailand but still faces three others brought by people accused of criminal activities in his investigative reports.

"Criminals do not have to pay costs when they lose and they continue to take cases which are so ridiculous in their nature that they are not even worth spending the cash to defend," he said.

"I have worked as a journalist for over 25 years in Britain, the United Kingdom and Australia without being sued once...in many ways foreign criminals (in Thailand) will be seen to have won this battle but they have not won the war."