A Christian private school principal has put teenage climate change activist Greta Thunberg on blast in a newsletter, labelling her a “little girl with mental problems”.

Rodney Lynn, head of Coffs Harbour Christian Community School sent out the letter to students and their parents on September 26.

News.com.au  reports that Mr Lynn urged his students to put their faith in God and “not in the predictions of a little girl”.

He did not refer to Thunberg by name, but he wrote about “a little girl from Scandinavia” who was promoting “doomsday waffle talk”.

“No one knows when the final wind up of the world will be,” he wrote. “Jesus said no one, only the Father God, knows about that day or hour.”

He said the 16-year-old was a “little girl with self-declared various emotional and mental problems that she thinks give her a special insight into a pending doom”.

“She says she is anxious. You too can be anxious. My life experience has taught me that doomsday predictors are just attention getters,” wrote Mr Lynn.

The letter was published the same week Thunberg made a passionate speech at the United Nations in New York as she held leaders accountable for their actions.

Now, young people around the world are taking action, as they demand for more to be done on climate change. But Mr Lynn questioned their actions.

“You can skip school. Hold up a piece of cardboard in the streets and call out for the government to ‘do something to stop it all happening’ … really???” he wrote.

“Do not be afraid. Your world’s future is in the hands of God, not in the predictions of a little girl and false prophets.

“God’s promises have never failed yet,” he wrote before signing off.