Authorities are investigating a wastewater discharge from the Rangiora Wastewater Plant that has flowed into the Southbrook, Cam River and Kaiapoi River.

Environment Canterbury said the incident occurred on Wednesday and involved an estimated 120 cubic metres of wastewater entering the waterways before moving downstream into the Kaiapoi River mouth.

The public has been advised to avoid fishing or collecting shellfish from the Kaiapoi River mouth until further notice while the investigation continues.

Officials say further updates will be provided once testing and assessment work is completed.