A Sydney father who defamed a school principal in an email attacking her competency has been ordered to pay her $82,543 in damages.

NSW Supreme Court Justice Henric Nicholas today handed down his verdict in favour of Jennie Ryan, the 61-year-old principal of Beecroft Public School, in Sydney's north-west.

She had sued Rajaratnam Premachandran for defamation over an email he sent to fellow parents in April 2008.

The email, which he also copied to the principal, was entitled "Call for Mrs Ryan to quit" and alleged she was not fit to remain as principal.

Mrs Ryan's lawyer, Sue Chrysanthou, had submitted Mr Premachandran then ran a year-long malicious "campaign" against the well-respected and "decorated" principal.