"I'm not a man of big means but I just got carried away," he said.
"My wife doesn't know what I paid for it yet. I said 'a few thousand' and she thought it meant $2000 and went off at me. I don't know how I'm going to explain that it was actually $16,000..."
The gum was collected by the anonymous seller's Scottish grandfather who worked as a limber in the kauri logging industry around the 1890s.
During his job of removing top branches before the giant native trees were felled, the worker reportedly found the huge piece.
Kauri gum forms when resin exudes from cracks in tree bark and hardens by exposure to air.
Pieces collect in the axils of branches and in the debris at the base of the tree. The colour of the gum ranges from pale yellow to reddish-brown and even black.
Mr Grigg said two other large naturally bled nuggets have been kept by family.
The buyer said his grandfather worked at the kauri mill on the Kaipara and he's been interested in the industry ever since.
He has collected forestry tools, included saws, and has now turned his attention to gum.
"This piece of kauri gum is the finest I've ever seen," the man said.
He entered a bidding war with some overseas buyers, which spurred him on.
"I over-stretched myself a bit, but I want this stuff to remain in New Zealand -- it's part of our heritage."
