A motorbike rider killed in a South Island school's fundraising event was a "husband, father, colleague and friend" whose death has devastated organisers, says the school.

The man, whose name has yet to be released, died on the Lake to Sea Trail Bike Ride in Otago - a Tokomairiro High School PTA fundraising event, which is in its 18th year.

It was one of three crashes during the event, with two others seriously injured and another eight people also hurt.

"Our hearts go out to this man's family and friends as they deal with the loss of their husband, father, colleague and friend," Tokomairiro High School PTA chairwoman Lynne Johnston said in a statement.

"Our focus at this time is to support the family and those affected by this tragic accident."

Johnston said two other riders had to be airlifted due to remoteness with injuries; both of these were the result of single bike accidents.

Other injuries dealt with ranged from a broken collar bone through to a sprained wrist and cuts and grazes.

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.

"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."

Earlier, it was reported a man had died and another 11 people were injured after three crashes in a well-known school fundraising event in the South Island - and a local mayor has spoken of the devastating impact on families and friends.

Police confirmed that a motorcyclist was killed in one of the three crashes during the Lake to Sea Trail Ride event in Otago on Saturday.

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.

"The Serious Crash Unit attended and examined the scenes," police said in a statement today. "The death has been referred to the coroner and enquiries are ongoing into the
circumstances of the crashes."

Clutha Mayor Bryan Cadogan has voiced his support for the family and friends of those affected by the "devastating" impact of several crashes in the Milton area.

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, which is a fundraiser for the Tokomairo High School, offered a 140km 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 has been limited due to poor cellphone coverage in the area.

A St John spokeswoman confirmed 11 people had been injured.

The seriously injured were flown by helicopter to Dunedin Hospital.

Three others in a moderate condition were taken to hospital by ambulances. One person was transported from the scene in a private vehicle with moderate injuries.

Five others were treated at the scene for minor injuries.

The St John spokeswoman said the callouts related to multiple crashes.

The 18th Lake to Sea Trail Ride is a popular fundraiser for Tokomairiro High School in Milton, about 50km south of Dunedin.

The 140km trail ride for off-road motorbikes and ATVs goes from Milburn to Toko rivermouth – and back again – through a mixture of forestry and farmland.

Social media posts after the fatal crash have said there had also been some "bad crashes" at last year's event.

Organisers have been approached for comment.

Marshalls had been placed at junctions and road crossings, the event organisers said in pre-event information.

Organisers try to vary the track every year, but need the permission of various landowners.

Weather on Saturday had seen a high of 16C with light rain showers.