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Mr O’Brien said he had had a "wonderful leadership journey" at the Dunedin Catholic secondary school and had enjoyed fantastic support from staff, pupils and the parent community.
The 54-year-old said his decision was met with surprise from many at the school.
But after nine years at the college, and almost 20 years as a principal, he said it was time for new horizons.
He said a few health issues over the past couple of years had also played a part in his decision.
"I have loved my time as a principal, but the job is very demanding and those sort of things do enter your thinking about the time you have left and how you’re going to spend it.
"I just need the change and to reignite the fire and passion.
"I guess I’m throwing it to the universe a little bit. I’ve got a couple of things in my mind, but nothing definite. I’ll really start thinking about where I might go now.
"I’m looking forward to a little hiatus, as long as Mrs O’Brien will let me."
He said two things he was certain about was he would remain in Dunedin and it was unlikely he would take up another principalship.
"It would be hard to top Kavanagh.
"I feel like I’ve achieved all I wanted to achieve."
By signalling his intentions now, Mr O’Brien hoped it would provide certainty
and allow enough time for the board of trustees to find a replacement principal.
Mr O’Brien’s career started in 1985 at Rosmini College, in Takapuna.
His first principalship was as acting principal at Hato Petera College in Northcote, and went on to become principal at Cullinane College in Whanganui before coming to Kavanagh College.