Greta Thunberg has been savaged by a Christian private school principal who called her a "little girl" with "mental problems".

Following her emotional speech at the UN, the head of Australia's Coffs Harbour Christian Community School, Rodney Lynn, wrote to parents and students with a blistering message - don't listen to her "doomsday waffle talk".

Lynn said Greta was a "little girl with self-declared various emotional and mental problems that she thinks give her a special insight into a pending doom of 'climate change'".

"My life experience has taught me that the doomsday predictors are just attention getters," 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."

However, some Christian leaders have spoken out to criticise Lynn's remarks.

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Dr Greg Jenks, the Dean of Grafton Cathedral, said Lynn's views were a "horrible expression of Christianity", the Daily Mail reports.

Lynn's comments have also been criticised on social media.

"Enough of this old generation. At least the kids doing the strikes are thinking more about the world than themselves as individuals," one person wrote on Facebook.

"It's nice to know what he thinks of kids on the spectrum. I will be crossing this school off my list," another said.

Gretta has continually come under fire ever since she became a figurehead for the environmental movement.

Earlier this week, Top Gear host Jeremy Clarkson launched a tirade against the 16-year-old climate change activist, calling her UN appearance a "full-on adolescent meltdown".

"I'm sorry Ms Thunberg, but if you're going to lay into my generation, you must accept it when I lay into you and yours," he wrote. 

"We gave you mobile phones and laptops and the internet. We created the social media you use every day and we run the banks that pay for it all," he added. 

"So how dare you stand there and lecture us, you spoilt brat."

The controversial television presenter demanded Thunberg go back to school and study hard in her science classes. 

"Many thousands of people who you had the temerity to blame this week are trying to do exactly what you want," he wrote.

"So be a good girl, shut up and let them get on with it."