A hole has appeared behind the end of the German wall at the eastern side of L'Ancresse, near the slipway.
The States of Guernsey has fenced the area off, and work will start next week to fill it.
Under the latest States resolution, Environment and Infrastructure are to keep repairing the crumbling World War Two defence work for a number of years.
A £150,000 repair plan was agreed in April.
Members of the public are being asked to avoid the area and not pass the barriers.
Guernsey "largely unscathed" by Storm Austen
"No serious incidents" were reported in Guernsey during the first storm of the season.
Commodore Clipper breaks mooring ropes in Storm Austen
A video circulating on social media shows Brittany Ferries' Commodore Clipper drifting in stormy weather.
70 mph winds batter Guernsey
Alderney records even stronger gusts of 75mph from Storm Austen.
Haut Du Mont: Jury fails to reach verdict on third gas worker
A jury has failed to reach a verdict on a third gas worker accused of unlawfully killing ten people following the explosion at Haut du Mont nearly four years ago.
Schools in Guernsey to close for Storm Austen
The States has taken the decision to close schools as Storm Force 10 winds are forecast for the Friday commute.
States of Guernsey 'carefully monitoring' forecasts for Storm Austen
A decision is expected later today (22 October) on whether to suspend public services like schools and buses.
Haut du Mont: Gas workers convicted of manslaughter
Two gas workers have been found guilty of unlawfully killing ten people following a fatal explosion in St Helier in December 2022.
Guernsey government websites go offline amid security concerns
Many dot gg websites are inaccessible, following international warnings about a security weakness in web software.
