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.
New pontoons in St Peter Port Harbour
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Guernsey's latest crime trends revealed
Guernsey's crime rate is virtually unchanged, but some driving offences have increased significantly.
Man arrested over 'threats to kill' in Guernsey
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Jersey and Guernsey both lift trophies in Siam and Muratti weekend
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Men pretending to be police arrested for £10K Guernsey convent fraud
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Channel Islands' 10 wealthiest residents revealed
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85 home planned for Forest site in Guernsey
The vast majority at the Mallard would be classed as affordable.
Sewage outfall failure on Guernsey's east coast
Attempts will be made to repair the fault that has seen untreated sewage enter the sea at a local beauty spot.
