A St John spokesperson said the ambulance service received a call at 8.26am relating to the bus incident.
One ambulance attended the incident and treated a patient in minor condition, taking the person to Whangārei Hospital.
A spokesman from bus company Ritchies said he was trying to get hold of police to find out what had happened.
Jolt cafe supervisor Hiroo Fukuoka said a customer came in just before 9am to say an incident was unfolding.
Staying within the cafe, which was across the street, Fukuoka said he saw an ambulance turn up soon after but couldn't see inside the bus.
A nearby resident, who wished to remain anonymous, said police cars were parked at both ends of the bus when she first witnessed the incident.
"I noticed there was a plain cop car in front of it and two cop cars behind it. By the time we looked out there, the driver seemed to be sitting in the bus and within 20 minutes, an ambulance turned up and we know that there's regular school buses that stop there."
A senior staff member at a Whangārei secondary school said they first learned about the incident from the Ministry of Education, who were contacting schools to piece together this morning's incident.
A police spokesperson confirmed officers attended the incident on Maunu Road in Whangārei this morning after being called by someone worried about the bus driver.
"A small number of occupants who were on the bus have safely left the scene and the bus driver has been spoken to by police and is being medically assessed."
Bruce Clarkson from Titoki was in Maunu for a dermatology appointment and was about 40 metres away from the bus as it pulled up before 8am today.
Clarkson said two students, aged about 14, tried to get on the bus to join two others who were already on it, but couldn't as the driver became unsettled.
"[The students] tried to open the door and it wouldn't open and this bus driver just went absolutely ballistic, that carried on for a good quarter of an hour," he said.
"[The driver] was just waving his arms, shouting and yelling. I'm fairly deaf and even I could hear all the commotion."
Clarkson then went to get a medical professional from the nearby Tui Medical Centre, which was when the police were called.
Clarkson said he didn't think the students were in any immediate danger but noted they seemed shaken by the encounter.
"[The students] were standing there with their arms out, they couldn't work out what the hell was going on," he said.
"It was a bit of weird sight, it's not something you see every day of the week."
