UPDATED: 8.24pm  Firefighters have contained a series of fires along a south Canterbury river.

Fires in the dry riverbed of Selwyn River, and other rivers either side of State Highway One south of Burnham are believed to be the result of arson.

A police spokesman said officers are speaking with people in the area.

Southern Fire Communications Duty Shift Manager Brent Dunn said crew will stay overnight until all fires are completely out.

"The main fire to the east of State Highway One is approximately 350 by 70 metres square the other smaller fires are about 20 to 100 square, each of those."

Residents near Selwyn River who were earlier evacuated, have now returned home.

It took four fire engines, 10 water tankers and three helicopters to bring the fire under control.

No buildings have been burned, and no one is known to have been injured.

Police are calling for sightings of a 4WD vehicle that was seen in the riverbed close to the vicinity of the fires.