Seven people had minor injuries and 10 moderate.
Two people were airlifted to Waikato Hospital and three others have been taken by road - all moderately injured.
The remaining patients would be taken to either Te Kuiti or Waikato Hospital.
“It appears the road is blocked while emergency services respond,” police said.
Waka Kotahi tweeted that the crash was near Fullerton Rd in Waikato.
Delays in the area are likely and diversions may be required, the transport agency said.
Fire and Emergency NZ said they attended a crash at about 1.55pm. A spokesman directed further questions to St John and police.
A witness told the Herald a rescue helicopter was on the scene, as well as at least one ambulance, two police cars and firefighters using the jaws of life to help free trapped people.
An employee of nearby Abseil Inn, Helen Anderson, said the crash happened outside another nearby business.
”There are still quite a lot of flashing lights across the road,” Anderson said. ”First I knew of [the crash] was when the sirens were coming around the junction.”
Police inquiries are ongoing.
- More to come.
Contact the newsroom at newsdesk@nzherald.co.nz