The man hit the window of the Red Bus at the intersection of Ruskin St and Antigua St, in Addington, about 7.30pm on Friday, a police spokesman said.
He was taken to hospital with serious injuries.
There was “quite a bit” of damage done to the bus itself, the spokesman said.
Red Bus chief executive Paul McNoe said he understood the cyclist was in a cycle lane when he turned into the path of the bus.
“The driver did the best he could to avoid it, but he wasn’t expecting a sharp turn to the right,” McNoe said.
Camera footage on the bus would be reviewed and given to police, McNoe said.
A condition update was not available.