The accident happened west of Victoria midday Wednesday when the driver apparently over-compensated while making a lane change, the Royal Canadian Mounted Police said.
The truck skidded across three lanes and struck a roadside concrete barrier and overturned, the Victoria Times Colonist reported.
It narrowly missed striking any other vehicles and the driver sustained minor injuries, the RCMP said.
British Columbia provincial environment officials and hazardous materials specialists were summoned and managed to contain the spill to the road and shoulder, officials said.
A private environmental cleaning company was contracted to collect the sewage, the report said.