The three New Zealanders killed in a bus crash in Kenya were on a volunteer mission with a Tauranga school.

Brian Johnston, a Whakatane anaesthetist, his wife Grace, and former Bethlehem College student Caitlin Dickson died when the minibus they were travelling in lost control in heavy rain, rolled and went into a ditch.

Bethlehem College principal Eoin Crosbie told Radio New Zealand the trio were part of a group of seven adults and 12 students from the college in Kenya to do volunteer work.

Mr Crosbie said the accident happened on Tuesday morning (local time) as the group was heading back to the village of Mahanga, in Kenya's western province.

The driver of the minibus was also killed in the crash.

"Other members of the team, students and some teachers, have suffered injuries and bruising and broken bones and they're all at hospital," Mr Crosbie said.

The injuries were all minor.

"We've visited families of the deceased in the first instance and spent our time contacting the parents of students on the trip," Mr Crosbie said.

It was the school's third trip to Kenya. The group left New Zealand on December 28 and they were helping build classrooms in an under-privileged village.

The Bethlehem College Missions Facebook page has been inundated with messages of support for the families of those who have lost loved ones.

Bay of Plenty District Health Board chief executive Phil Cammish said Dr Johnston and his wife leave behind a large family and a number of grandchildren.

Dr Johnston was described by his colleagues as "the nicest person you would ever meet" and someone who "would only ever give, never take", Mr Cammish said.

The Ministry of Foreign Affairs and Trade (MFAT) says the New Zealand High Commission in Pretoria, South Africa, is providing assistance and support after the accident.

Bethlehem College is an independent Christian school.

Its website says it has a roll of about 1500, with pupils ranging from new entrants to year 13.