High flood waters have washed away one of the 34 piers holding up the 610-metre long rail bridge over the Rangitata River.
It resulted in the steel bridge spans sagging, a KiwiRail spokesperson said.
All trains in the area have been stopped.
"KiwiRail engineers are currently 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 cannot comment on how long it will take to repair the bridge at this stage or if there will be further damage to the bridge. "
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."
