A person with moderate injuries was taken to Whanganui Hospital after a single-vehicle crash on State Highway 4 near Upokongaro.
St John said they responded to the incident at 1.16pm on Thursday with one ambulance and one manager.
They treated three patients at the scene, one in a moderate condition who was transported to Whanganui Hospital, and two in a minor condition.
Police said emergency services blocked both lanes of the road while they attended the scene.
Fire and Emergency New Zealand shift manager Murray Dunbar said no one was trapped in the car as a result of the crash, and a fire crew stood by and assisted police and St John at the scene.
