Former principal Peter Malcolm, who headed the school from 1984 to 2004, spoke on behalf of the manuhiri (visitors).
The event's cake was officially cut by foundation staff member Marney Bradley and foundation pupil Cheryl Dawson.
A commemorative kauri tree was planted to join two other kauri at the college.
The first was planted at the school opening, the second at the 40th anniversary.
Mrs Kirk said Saturday's dinner and dance event at ASB Arena was "absolutely superb".
Guests were entertained by a line-up of former students, co-ordinated by long-serving staff member Mac McGregor, who has been at the college for 35 years.
"Every singer there had a connection to the school. From the first to the last song, the dance floor was completely full."
Foundation pupil Gary Chester met up with five former classmates and two of his former teachers.
Mr Chester said the school looked completely different now to when he started in 1965.
"It's just about unrecognisable. The first year, we had 200 people and now there's 2000. It's a much bigger school than it was. They wrecked our hockey field and put a whole lot of new buildings on it."
Mr Chester said the event went well and he had particularly enjoyed the music from the former students.
Former principal Peter Malcolm enjoyed the event - although he had to keep asking people not to call him "sir" or "Mr Malcolm".
"Catching up with former students and staff I've been with over the years, that's the best part. That's what reunions are all about."
Mr Malcolm said it was pleasing to meet up with so many people who had good things to say about their time at Otumoetai College.
"That certainly gave me a bit of pleasure. There were lots of comments from people who said they made a lot of friends at the school and got a really good education."
Current Year 12 student Wisdom Toki, 15, was part of the kapa haka group that welcomed the visitors to the school marae.
"It was exciting welcoming all the ex-students back on to the marae. There were many who hadn't seen it before because it wasn't there when they went to Otumoetai," he said.