The bus was travelling at 40km/h to 50km/h behind another bus near Mossburn when it slid and spun 180 degrees before coming off the road backwards, rolling onto its side and hitting a power pole.
While the cause of the crash was yet to be determined, police said road conditions were treacherous at the time.
The 37-year-old driver was from China and had been living in Auckland for the last five years.
"He is an experienced bus driver and is assisting police with their enquiries.''
The New Zealand Transport Authority attributed a "light shower of rain'' about 8am to the rapid deterioration of road conditions in the area.
Southland area manager Peter Robinson said the roads were inspected at 6am and 7am and no obvious ice issues were evident on the section where the crash happened.
Southern Lakes Helicopter pilot Mark Deaker, who attended the crash, said the roads were like a skating rink.