A Sydney father who sent a "malicious", untrue, defamatory email in a bid to force a school principal to resign has been ordered to pay her $82,543 in damages.

Justice Henric Nicholas concluded the 61-year-old principal, Jennie Ryan, had a "settled reputation for competence, honesty, and trustworthiness" and colleagues held her in high esteem.

But he said Rajaratnam Premachandran, who had children at her school, had made grave allegations of "incompetence, dishonesty, untrustworthiness" against Mrs Ryan.

"I am satisfied his allegations were groundless, and were the product of fantasy fuelled by enduring ill-will towards (her), and his desire to discredit her and bring her down," the judge said.

The principal of Beecroft Public School, in Sydney's northwest, sued Mr Premachandran in the NSW Supreme Court for defamation over the email he sent to fellow parents in April 2008.

Justice Nicholas concluded the email carried a string of defamatory meanings and rejected all of Mr Premachandran's defences, including a contention the claims were true.

During the hearing, Mrs Ryan's lawyer, Sue Chysanthou, said the father ran a year-long "malicious" campaign against the principal, including writing to the education minister and the Indian High Commissioner.

Ms Chysanthou said Mr Premachandran had blamed the principal for his child's poor performance in selective-school tests.

"He just spewed out his rage on the page," she said.

The judge noted Mr Premachandran was well educated and had no difficulty with the English language.

"I have no doubt that he well knew the ordinary meaning conveyed by the terms of his emails, and of the likely impact of them," he said.

His motive had been to force Mrs Ryan to resign by "discrediting and denigrating her before the recipients", the judge found.

"Put another way, I am satisfied that his dominant motive, by reason of his dislike and anger towards her, was to so injure (her) that she would resign," Justice Nicholas said.

He found the email had caused Mrs Ryan "great distress and emotional upset, and that her sense of hurt continues to the present time".

This had been seriously exacerbated by the malice of Mr Premachandran, demonstrated by his other emails and letters, his failure to apologise, and his contention the defamatory statements were true, the judge said.

The question of legal costs will be decided on another date.