Hato Hone St John sent three road ambulances and an operations manager to the crash and treated six patients before transporting them to various destinations.
One patient was flown to the Waikato Hospital Emergency Department in a serious condition. Four others were taken to the Thames Emergency Department.
SH25 remained closed on Thursday evening and commuters were advised to expect delays or use an alternative route.
A Fire and Emergency NZ spokesperson earlier told the Herald two people were trapped in their vehicles and attempts to extract them were under way.