Wearing seatbelts may have saved four students from injury when their school minivan smashed into a tree in the Waikato this afternoon, police say.

Seven students from Putaruru-based Maori immersion school Te Kura O Te Kaokaoroa O Patetere were in the van, travelling as part of a convoy on Maungatautari Road, 10km southeast of Cambridge, when the vehicle skidded on the wet road, careered 30m over a grassy strip before crashing into a tree shortly after midday.

The 39-year-old driver, from Tokoroa, was trapped for more than half an hour and suffered serious leg injuries and blood loss.

He was tonight in a comfortable condition in a ward at Waikato Hospital, hospital spokeswoman Mary Anne Gill said.

Sergeant Warren Shaw of the Te Awamutu Strategic Traffic Unit said the crash happened on Maungatautari Rd about 12.35pm.

"It appears the driver of the school bus-van lost control of the vehicle on a wet road and the vehicle left the road and crashed into a tree," Mr Shaw said.

Three other students travelling in the van were unhurt but shaken by their ordeal.

Mr Shaw said the crash could have been a lot worse and was a timely reminder for drivers to drive to the conditions.

Maungatautari Rd remains open.