"He suffered very serious head and neck injuries and had to be resuscitated and transported to hospital.
"He had a cardiac arrest from his injuries," Mr Devanney said.
Details around what had happened at the BMX track were sketchy, he said, but it appeared the man hit his head against an object while riding.
The Whangarei BMX Club was aware of the accident but members did not know how it had happened.
Club president John Romer said it had taken place just hours after club members, who had a fun day on Sunday afternoon, left the tracks.
The seriously injured officer was not a club member, he said.
"We run a tight ship in terms of safety when we're out there.
"Things like first aid, posting officials around the corners, because of a lot of young kids right through to adults take part," Mr Romer said.
"It's horrific to see anyone hurt himself ... It's [BMXing] quite a dangerous sport and unfortunately when we're not there it's an open track, just like the skate park in terms of safety."
The Whangarei District Council, which owns the tracks, was told about the accident yesterday and spokeswoman Ann Midson said all inquiries were being dealt with by police.
Northland District Health Board spokeswoman Paula Martin said the injured police officer was admitted in the intensive care unit in a serious condition.
Northland police spokeswoman Sarah Kennett said the officer's family was asking for privacy.
