A section of the metre-high concrete base of the Prince Street seawall between North Cronulla and Elouera beaches has been undermined by pounding waves over the last few weeks.
Remnants of a timber seawall which pre-existed concrete structures have also been exposed.
The impact of two east Coast Lows in a fortnight includes extensive erosion to beaches and damage to retaining walls from Greenhills to Cronulla.
The Esplanade between Cronulla and North Cronulla beaches will remain closed for up to two weeks while engineers check whether any remediation work is necessary for the structure or damaged cliff face and private property.
The concrete base of "The Wall", which is normally covered by sand, is fully exposed at low tide.
At low tide on Thursday morning, there was no sand beneath a section of the base.
The southern corner of North Cronulla beach has disappeared, leaving a steep drop and what used to be a sandy access path is now a mass of boulders.
Both corners of Cronulla beach have suffered severe erosion.
A Sutherland Shire Council spokesman said, "The combined impact of recent heavy rains, strong swells and significant high tides have caused significant sand erosion across a number of shire beaches and damage to cliff faces, retaining walls and other infrastructure immediately adjacent to the Esplanade.
"Council is currently taking a number of proactive measures to address any potential threat to public safety, with access tracks to beaches from North Cronulla to Greenhills set to remain closed for the foreseeable future owing to the significant volume of sand erosion that has occurred across these beaches.
"Council is also working to remove fencing which has been damaged as a result of sand erosion, with an excavator to be employed to maintain essential beach access for council lifeguards."
The spokesman said the Esplanade between North Cronulla and South Cronulla Beach would remain closed for up to two weeks to allow geotechnical engineering inspections and any necessary remediation works to be carried out to damaged cliff faces and private property.
"Council will work with independent contractors and neighbouring landowners to ensure that these works are conducted in a timely fashion and that public access can be restored as soon as it is safe to do so," he said.
"Council also continues to assess damage to the Prince Street seawall between North Cronulla and Elouera, with further works to be conducted to reinforce any sections that have been rendered unstable.
"Local residents are advised to avoid this area as sand erosion has also exposed remnants of the old seawall, which poses a potential hazard for surfers, swimmers and other beachgoers.
"Council wishes to thank local residents for their patience as these works are conducted, and seeks to remind all local residents to exercise caution and follow directions from council staff and site signage employed to keep these sites safe."
