A Northland man who accidentally shot himself with a small cannon was a trained firearms instructor who had been around weapons all his life.

Derek Allan Kelly, 74, a gun collector from near Dargaville, was killed on Sunday afternoon by what police described as a small, single-shot cannon.

Police spokeswoman Sarah Kennett said Kelly was killed when he attempted to move the cannon from the tray of his ute and it accidentally discharged.

Kelly died at the scene in front of his wife who was "very distressed".

Kennett said the barrel of the cannon was about 40mm in diameter.

Police were still investigating the circumstances and couldn't reveal more because the matter was with the coroner, she said.

Barry Shine, a friend of Kelly's for 20 years, described him as an "upstanding citizen".

"You'd go a long way to find as nice a guy as Derek," he said.

"It's an absolute tragedy, and my thoughts are with his wife and family."

Kelly was a trained firearms instructor, who was heavily involved with the local community, Shine said.

"He's been around firearms all his life."

Kelly was a member of the New Zealand Antique & Historical Arms Association, and proficient with pistols and bolt-action rifles.

Shine could not recall Kelly using or having any cannon-type weapons.

He was a former dairy farmer, who had also worked driving bulldozers and heavy machinery in Australia.

"He was very helpful, very community-minded. It's a tragedy to lose someone like him," he said.

Another mate, Neil Black, described Kelly as "a great guy".

"He was a good family man and a community worker," Black said.

"He had a big collection of firearms - all sorts of things."

Black was not familiar with the cannon involved in Kelly's death, but said: "There are groups that build these sorts of things, they're perfectly legal."