Mr Key told the audience it was a special day for him to be in Dunedin, as it was two years ago to the day that his Government was sworn in.
He congratulated the Balmacewen Intermediate kapa haka group for their singing and later spent about 15 minutes talking to the pupils and signing their caps and other items of clothing. As the pupils left, Mr Key was seen giving high fives to many of them.
He praised NHNZ for its innovative work which kept its programmes being shown on "dozens and dozens" of television channels in 180 countries around the world.
"This is a remarkable company that has its products all around the world. The secret of New Zealand is its creativity and brilliance of its people, and that seems to be the ethos of this company."
It was not as though NHNZ did not face competition internationally against companies with deep pockets, he said. But the winning of contracts and the ongoing success was a credit to all of the people who worked for the company.
Mr Key's family had a new high-definition television and it was great watching the documentaries on that, he said.
It was a privilege to open the building as it was a sign of the success and commitment of NHNZ, he said.
NHNZ chief executive Michael Stedman said the company had started in a small way in Dunedin about 35 years ago. Now, it had a company in Singapore, had a significant shareholding in South Africa's largest wildlife-documentary maker, had a base in Washington DC and was the largest Western producer of documentaries in China.
It was setting up an office in Abu Dhabi and was "dipping its toes" into the Australian market.
Mr Stedman praised his staff, who he described as a "special resource" for their hard work in ensuring the success of the company.
They were a "remarkably talented" group of people, he said.
Dunedin Mayor Dave Cull said NHNZ had outlasted Dunedin's early IT, film and entertainment sector that started in the city many years ago and ended with the closure of Television New Zealand's studios in 1987.
"I had personal experience with that as I lost my job at the time. But isn't it great that a company in Dunedin [has] had to shift as it was too big for its premises? They were busting at the seams and had to move to continue to grow."
NHNZ was successful because it was ahead of its competition in all aspects, he said. It was an important part of the city.
