Two other riders with injuries had to be airlifted due to the remoteness of the area; both of these were the result of single-bike accidents, she said.
Other injuries dealt with ranged from a broken collar bone through to a sprained wrist and cuts and grazes, she said.
The track is 140km long and is designed to cater for a variety of riding levels with sections specifically for junior riders and others for experienced riders.
This year there were 870 riders taking part.
Johnston said: "The Tokomairiro High School PTA community thank all those members of the community for their support of this event, to St John's for their tireless work, and to the rescue helicopter and police for dealing with this very sad situation."
Emergency services responded to the three crashes after two distress beacons were activated in the area about 12.10pm. Two of the three crashes occurred within 150 metres of each other.
Police say the Serious Crash Unit attended and examined the scenes.
"The death has been referred to the Coroner and enquiries are ongoing into the circumstances of the crashes," a spokesperson said.
Clutha Mayor Bryan Cadogan has voiced his support for the family and friends of those affected by the "devastating" impact of the tragedy.
A St John spokeswoman confirmed that a total of 11 people were injured at the event, with five hospitalised.
Two people had been transported to hospital by helicopter with serious injuries.
''My thoughts go out to the family and friends,'' Cadogan said, and it had been ''devastating news'' to hear about the crashes and their outcome.
The 18th annual event offered a ride from Milburn to Toko Mouth and return with a mixture of forestry and farmland.
A "less challenging course" of about 80km is available for less experienced riders, while riders under 12 must be accompanied on the course by a parent or guardian.
Helmets and sturdy boots are compulsory. Bikes must be mechanically safe, organisers say, with good tyres and have a 60km fuel range, plus a can for lunchtime refuel. Scrutineers check over the bikes before going on the course.
Offroad bikes and ATVs could be used in the event, but full information about details of the respective separate crashes was not immediately available.
A police spokeswoman said that information from the crash sites was limited due to poor cellphone coverage in the area.