It did not look like she checked for traffic before stepping onto the road, he said.
"There was a huge bang. She flew about three metres through the air and then landed in the gutter.
"I think it was just one of those things where she just wasn't expecting a bus to be coming along,'' the witness said.
The impact knocked her out for about two minutes, and then she started "howling'', he said.
"She was conscious, she started to struggle and things like that.
"There was a lot of blood.''
A doctor who was nearby stayed with her until ambulance staff arrived.
The woman appeared to be aged in her 30s or 40s. She was not wearing earphones at the time of the incident, he said.
Ambulance central communications team leader Marie Willis said the woman was transported to Wellington Hospital in a serious condition.
Police have closed one lane of Manners St while they investigate the incident. The Go Wellington bus involved was still at the scene.
NZ Bus said it was was notified of the incident immediately through the driver contacting the central communication control team.
"Regional Operations Manager Richard Graham was on Manners St within minutes to help police and ambulance staff, the serious crash unit investigation and to support the Go Wellington bus operator,'' a spokeswoman said.
"In circumstances such as this, the driver is naturally very upset and will be checked over for any signs of medical injury or stress.
"The driver is not expected to work for the rest of their scheduled shift and will be off work for the foreseeable future.''
The driver would also receive counselling as well as support from family and colleagues, and would be monitored on their return to work.
Someone has been run over by a bus on Manners Street. They really should slow down more. pic.twitter.com/2jzYtgMZ99
