It was speaking after at least fifteen Chinese human rights lawyers and activists were formally arrested on suspicion of "subversion of state power" or "inciting subversion of state power".
Stuart Windsor, from Christian Solidarity Worldwide, said that hundreds of people are missing: "Many, many defenders of freedom of belief and freedom of expression, who've defended Christians or who've been Christians, and who stand for faith and justice have disappeared and are still missing."
Those missing include prominent human rights lawyers Wang Yu and Li Heping, and the Christian activists Liu Yongping and Gou Hongguo.
Wang Yu was the first rights lawyer to be detained in what has been called the "709 Crackdown".
The 15 people accused of these charges are among more than 300 lawyers and activists, as well as relatives and associates, who were interrogated, detained, imprisoned or disappeared between July and December 2015.
Mervyn Thomas, CSW Chief Executive told Premier: "It's not unusual for human rights lawyers and activists to be arrested or to even disappear - but in these numbers it clearly means there's some kind of concerted effort to silence these people."
He added that people simply do not return home one day: "They just disappear initially and nobody knows where they are.
"They don't tell the family, they don't tell anybody where they are."
CSW's Chief Executive Mervyn Thomas said: "We continue to be deeply concerned about the formal arrest of lawyers and activists in China in connection with this crackdown on the rights defence community.
"Lawyers play an essential role in the defence of human rights. In China, lawyers and rights activists have been instrumental in defending the rights of religious communities and in calling for further improvement of the protection of freedom of religion or belief.
"We join international human rights organisations, lawyers, and other voices in the international community in calling on China to release those held without legal basis, to protect and uphold the rights of all those detained, and to guarantee the personal rights of lawyers when they perform their duties, as stipulated in the National Human Rights Action Plan of China."
Listen to Premier's Rick Easter speak to Mervyn Thomas and Stuart Windsor here:
