Belle Greve Bay sewage spill warning from Guernsey Water
Raw sewage was pumped into Belle Greve Bay after a technical fault, Guernsey Water said.
Signs were placed around the bay warning of a risk of sewage pollution after unscreened material was discharged for 40 minutes on Monday.
Guernsey Water said the "isolated incident" would be investigated.
Mark Walker, acting director of water services, said the fault meant sewage could not be discharged via the long sea outfall.
Mr Walker said the sewage may take up to 36 hours to disperse, although there was no evidence it was dangerous.