A young girl has been found in a suitcase in a bus luggage compartment, and a woman has been charged with ill-treatment of a child.

Detective Inspector Simon Harrison said police were called at 12.50pm on Sunday, after a passenger asked for access to the luggage compartment during a planned stop in Kaiwaka.

"The driver became concerned when he noticed the bag moving. When the driver opened the suitcase, they discovered the two-year-old girl," he said.

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Harrison said the little girl was reported to be "very hot", but otherwise unharmed.

She has been taken to hospital in a "minor" condition for an extensive medical assessment.

A 27-year-old woman was arrested and has been charged with ill-treatment/neglect of a child.

She is due to appear in the North Shore District Court on Monday.

"We would like to acknowledge and commend the bus driver, who noticed something wasn't right and took immediate action, preventing what could have been a far worse outcome," Harrison said.

"Police enquiries into this incident are ongoing and further charges cannot be ruled out. Oranga Tamariki has also been notified."

An Intercity spokesperson said the company was aware of an incident involving a passenger on one of its services earlier on Sunday.

"Police were called to respond, and they are now investigating the matter. No one was harmed during the incident and the service has resumed. As this is now an active police investigation, we are unable to comment further."