The Tweed Shire Council says the collapse of a 50-metre long section of a riverbank in Murwillumbah could compromise the town's flood levee.
The slip occurred at the southern end of the concrete wall section on Commercial Road.
Council engineer Danny Rose says emergency repairs will be done to stabilise the bank.
He says the damage was done several weeks ago.
"We were actually aware that there was a problem in that particular area immediately after the January flood," Mr Rose said.
"There were some signs of cracking, but because there was a lot of vegetation on the bank and we were waiting until the river went back to its normal levels and the area dried out a bit, which it hasn't really been able to do, we couldn't tell the full extent.
"But then last night the slip occurred.
"We'll be doling some immediate temporary works to try to amour that bank and make sure it doesn't get any worse.
"Our hope is that we can put these emergency works in place and that will buy us enough time to put a permanent solution in there without exposing the town to any risk.
"But we have let the SES know that there's been an issue with the levee so that if a big flood is forecast and does occur, we can keep a very close eye on that."
© ABC 2013
13:16 EST Sydney has endured a fairly consistent run of below-average overnight temperatures and rainfall so far this month, but conditions are looking set to change as school goes back next week.