Investigations are ongoing after multiple fires near Selwyn River, south of Christchurch.
A police spokesman said officers at the scene were speaking to people in the area.
Police are calling for sightings of a 4WD vehicle that was seen in the riverbed close to the vicinity of the fires.
The suspicious fires occurred at around 4pm in the Selwyn Riverbed near State Highway One just north of Dunsandel.
Residents living nearby had been evacuated, but have now been allowed back into their homes.
Four fire engines, 10 water tankers and three helicopters had been used to contain the fires.
A Fire Service spokesman said crews were still on the scene but the fires are now largely under control.
Fire shift manager Andrew Norris said the largest fire was 500 metres by 100 metres with two other smaller fires upstream.
Firefighters responded from the Dunsandel, Burnham, Rolleston and Lincoln stations.
No buildings had burned and no one is believed to have been injured.
Anyone who may have seen the vehicle or its occupants are urged to contact the Selwyn Police on 03 347 9248 or Crimestoppers on 0800 555 111.