A charity horse trek organised by the Multiple Sclerosis Society in New Zealand has been devastated by the loss of 12 horses after they took fright while resting in a paddock on the second night of the week-long trek and bolted through fencing.

The Multiple Sclerosis Great New Zealand horse trek in the North Island which set off fromUrenui 30km north-east of New Plymouth on 1 March, has been organised to raise funds to help finance further research into the disease.

The tragedy happened on the evening of the second day. According to reports in the New Zealand Herald newspaper the incident, which took place at around 10.30pm, occurred when 50 horses were spooked and panicked. In the stampede that followed 12 horses died or had to be destroyed after falling into ditches or down a steep hillside.

Many others were reported to be seriously injured and the search for the missing horses carried well into until the following day when the last one was found.

None of the riders involved in the trek were injured, although many of the distraught trekkers are to receive counselling from Victim Support volunteers.

More than 270 riders and their horses registered to take part in the seven-day trek, each paying 700 NZ$ (around £250).

The majority of the group have taken the decision to press on with the trek, which is scheduled to finish at the end of this week, although some are said to be too upset to continue and have returned home.

Trek organiser Trudi Jordan told The New Zealand Herald that the cause of the outbreak was a mystery, with theories ranging from traffic noise to an earthquake or lightning. “I don’t think we will ever discover what the cause of it was.”