Flood waters washed away one of the 34 piers in the 610 metre long rail bridge over the Rangitata River near Timaru.
This resulted in the bridge sagging and all trains in the area have been stopped.
A KiwiRail spokesperson said engineers were putting together a repair plan "but at this stage we have been unable to undertake a close physical inspection of the damage due to the high water levels".
"We will be able to move rail freight by road between Ashburton and our Timaru rail terminal (approximately 50kms) while the bridge is closed," the spokesperson said.
KiwiRail could not comment on how long it would take to repair the bridge or if there would be further damage to the bridge.
About eight freight trains travel across the bridge every weekday, and about five each weekend.
No passenger trains use the line south of Christchurch.
The Rakaia River and Rangitata River both peaked in the upper reaches early this morning, an Environment Canterbury spokesperson said.
The Rakaia rose to about 2100 m3/s and the Rangitata to around 1100 m3/s.
"These flows will pass through the lower reaches today," the spokesperson said.
"Overflows from either river are unlikely, but landowners adjacent to these rivers should monitor their own situation."
Environment Canterbury staff will monitor vulnerable locations to check for any issues during the day.
