The alleged assault happened just after 8am on Great North Rd in the suburb of Grey Lynn, police said.
An hour after an appeal for help, police said they were no longer seeking information on the man's identity.
"We acknowledge the public for a very swift response to our appeal this afternoon," police said.
Detective Senior Sergeant Martin Friend said the man got on the bus at a stop near King St this morning.
"He has sat down next to a high school student and touched her inappropriately," Friend said.
"This was witnessed by a passenger who quickly intervened and stopped what was taking place."
The man was told to get off the bus and exited at a stop near the intersection with Maidstone St.
"The witness did an incredible job in coming to the assistance of the student," Friend said.
The witness and another passenger supported the girl, walking her safely to school, he said.
Appropriate welfare support is being put in place for the student.
The witness took a photo of the man which police circulated.
