The rider of the bike who is in his 20s is understood to be uninjured.Cambridge Police posted on its Facebook page: "We would like to pass on our condolences to the family of the deceased and also the motorcyclist involved. It is awful for everyone involved.
"We would also like to say a big thanks to the crew and volunteers at Cambridge Motocross Club for their professionalism and help in the circumstances - you guys were awesome."
Firefighters performed CPR on the man at the event but it was unsuccessful.
Earlier today police appealed for anyone with photo or video footage of the "freak accident" to contact them and warned against posting it on Facebook.
Waikato police Detective Inspector Hywel Jones confirmed the man was a spectator.
However, he said he could not comment further because it was an active investigation between Police and Worksafe New Zealand which would continue tomorrow.
The NZ Motorcycling website said round two of the Cambridge Motorcycle Club MX competition was scheduled to take place on Sunday.
The gate of the Cambridge Motor X track was firmly closed this afternoon and the last utes carrying bikes drove off at about 4.30pm.
The Cambridge MX Club's vice president would not comment and a Motor X official at the track told the Herald they would not be making any comment as police were handling it.
The motor bike involved in the fatal crash was taken away on a ute earlier in the afternoon.
One neighbour recalled an accident happening at the track a decade earlier.
The club runs events at the track every Sunday of the month between March and September.
In a recent newsletter to neighbours the club described itself as being family friendly and "one of the most popular in New Zealand". It has 200 members.
