Witnesses reported seeing a "young girl" approaching the station in the Yobe state capital Damaturu around 7:00 am (0600 GMT) and refusing to be searched by security guards at the gate before blowing herself up.
"A private taxi... drove out of the park. As soon as the car came close, she detonated the explosives. Six people in the car were killed. She was also killed," bystander Sani Dankamasho told AFP.
Reports said the bomber was aged around 12.
The attack came just hours after United Nations chief Ban Ki-moon wrapped up two-day a visit to Africa's largest economy during which he hailed its "greater stability and peace" under its new leader as he commemorated a deadly 2011 attack on the global body by Boko Haram militants.
Police and witnesses reported a second attack 10 minutes after the bus park bombing, in which a suicide bomber managed only to kill himself and slightly wound a bystander on the outskirts of Damaturu.
"There were two suicide blasts in Damaturu today. The first one happened outside the motor park and the second one occurred in Pompomari on the outskirts of Damaturu. But only the bomber has died in the second blast," said Toyin Gbadegesin, a spokesman for Yobe state police.
"In the first blast outside the motor park six people were killed and 42 others injured by a female suicide bomber. The injured are receiving treatment in hospital."
No group immediately claimed responsibility for the bus park bombing but it bore the hallmarks of Boko Haram, which has in recent weeks used young women to carry out bloody suicide attacks in the restive northeast.
"The dead have been evacuated to a mortuary while the injured victims are currently being attended to by health and emergency workers," Yobe local government spokesman Abdulahhi Bego said in a statement.