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.
Landowners reminded to clear Guernsey's streams of vegetation
Guernsey Water say the island's 100 miles of fresh water douits are vital for water catchment.
Guernsey homelessness charity 'deeply concerned' at rising support need
In the first five months of 2026, At Home in Guernsey says the number of people it's supported is already nearing the total for the entirety of 2025.
Plans lodged to improve Guernsey's sea water Children's Pool
A planning application has been submitted to improve La Vallette slipway by the Children's Pool, following the damage caused by Storm Goretti.
States record £45M surplus, but still warn of Guernsey's funding gap
Guernsey's public finances turn in a surplus of £45 million, but P&R says the bigger picture shows a funding gap.
Warmest Guernsey spring on record
Guernsey has had its warmest spring since records began.
Parent challenges Guernsey school's attendance rule after Prom exclusion
A Guernsey parent says her son is devastated after being told he cannot go to his Year 11 prom because his attendance fell below the school's required threshold.
Survey to expose barriers to adult learning in Guernsey
The findings of a public survey will be used to decide what future qualifications should be offered to adults at The Guernsey Institute.
P&R President laments lack of progress as development agency chair resigns
Guernsey's senior politician says there has been a frustrating lack of progress on the regeneration of the island's two ports.
