The bus crashed on Riddell Rd and came within metres of hitting a house, said the home occupant, who did not want to be named.
The Glendowie resident described the sound of the crash that "just kept going".
He says it's a slippery corner and he had seen an accident there before.
He first thought a vehicle had driven into parked cars.
When he opened his front door and saw the bus sitting about six metres from his house, he went into "operation mode".
"First thing I saw were a bit of wires and it was about making sure they weren't electric. I could see that they were fibre. There still is petrol dripping out the bottom of it," he said.
"I knocked on the window and made sure the driver was okay. He was in a lot of shock, quite upset," he said.
"The other passenger was an elderly gentleman who had a gash on his chin. Probably worth about four or five stitches."
NZ Herald business reporter and witness Jane Phare said the bus driver was in shock after careering off the road in Glendowie.
"The pōhutukawa was demolished," she said.
"The front windscreen [of the bus] was smashed."
The bus drove through fibre lines and there was "quite a bit of oil" on the road.
Phare said the driver was taken to hospital and a single elderly passenger sitting in the back of the bus sustained a cut.
Three St John vehicles attended the scene in Glendowie and transported the two patients to hospital in moderate condition.
A police spokesperson said they were called to the scene near the Glover Rd intersection at 6.55pm where the passenger was being treated for minor injuries.
Fire and Emergency New Zealand also attended the scene.
