An alternative route through Ashburton since the closure of the main bridge is expected to be opened tonight.
A spokesman for the Ashburton District Council said they have assessed the bridge, located on State Highway One, on Tuesday morning and it will not be a band-aid type fix.
He said the bridge has sustained significant damage.
Ashburton Mayor Neil Brown said it is now looking like it will take more than a few days to fix.
"We've been up there on Thompsons Track, contractors up there believe they might have an access way open by tonight.
"They're working as hard as they can."
Brown said he is reasonably confident an inland route through Ashburton will be established on Tuesday night.
The Ashburton District Council made the announcement of the closure on Tuesday morning after days of heavy rain lead to flooding.
Located on State Highway 1, it is the main bridge across the Ashburton River, and an issue with piles has caused the bridge to start slumping.
Two to three months of rain has pelted Canterbury in the space of a few days in a deluge that has been labelled a one-in-100-year event.
Local authorities across Canterbury evacuated more than 300 properties amid wild weather that forced hundreds of residents to flee raging floodwaters threatening their homes.
Ashburton was one of the worst-hit areas, according to figures from Niwa, nearby Lismore received 238mm of rain in 2.2 days — the same amount it experienced in the previous 187 days.
Ten state highways were closed, while other roads were ruined and four bridges washed away.
A rare "red warning" was issued by MetService late last week, just the second time such a warning had been put out by the authority.
The red warning between Geraldine and Amberley lifted at 7pm last night but an orange warning from Amberley to Kaikōura was due to lift at midnight.