
Photo: Supplied/Jarrad Wilson
A police spokeswoman said emergency services were called at about 5.50pm.
Both lanes of the highway were briefly closed but police have reopened one lane under traffic management.
A passenger on the bus, Robert Farquhar, said it started having problems around Waitati but persevered for about another ten minutes before losing power.
"Gradually smoke started filtering into the cabin. No surprise we all hightailed our way off the bus to watch it slowly become engulfed in smoke and then flames," he said.

Fire fighters battle a blaze on a passenger bus on Dunedin's northern motorway. Photo: Stephen Jaquiery
Fire and Emergency New Zealand were also at the scene.
- Were you on the bus? We'd like to hear from you. Email reporters@odt.co.nz