The site accessed, www.sex.com, had "hard-core" pornography, but not illegal content, an agreed facts statement read.
After a report to the school from Netsafe, the board of trustees investigated and asked the principal for an explanation.
He admitted his actions were inappropriate and resigned on January 30, 2015.
In explanation, the principal said he suffered a devastating experience five years beforehand, leaving him with chronic depression. At the time he accessed the site he was having flashbacks, and was using the pornography to escape from the psychological pressure.
The man said his "cumulative actions" had a devastating effect on his wife and daughter, and put him at an extreme low. He expressed remorse, and said he wanted to get well a resume his career.
A psychologist said the man had a fragile mental state. She argued if he was named it would further impact his mental health and his already strained marriage.
The man was a good principal otherwise, his school said.
The tribunal said because the man lived on the premises and did not have private internet access, the boundaries in the case were particularly blurred. However, it said "for a teacher to use a school computer system to access pornographic material virtually always constitutes serious misconduct".
The tribunal therefore ruled serious misconduct in the man's case.
After taking into account his mental health and good record, and desire to get well, it censured the man and placed conditions on his teacher registration certificate for 18 months. He also had to be supervised by a psychologist and forewarn any new employees of his history.
An annotation would be added to his file, and he would pay the complainant -- the Education Council's Complaints Assessment Committee -- $1900 in costs.
His name would be suppressed to help his recovery.
