A water firm has been fined £10,000 after pumping out sewage eight times the level that can kill fish.

The pollution was so bad, Canterbury magistrates court heard, ammonia levels were twice the permitted level three days after the spillage in April.

The Environment Agency prosecuted Southern Water for allowing sewage to leak into a tributary of the River Rother from its sewage treatment works at Tenterden.

Southern Water admitted the offences and was fined £10,000. The firm also agreed to pay investigation costs of £7,295, plus £45,000 to the Environment Agency for clean-up costs.

The court heard discharges of treated sewage from the Southern Water site at Tenterden are allowed to enter the Newmill Channel under strict conditions set out by the Environment Agency to protect water quality.

The site also has a treatment facility for processing sewage sludge to create fertiliser for farmers.

But on April 1 this year, the Environment Agency was told by Southern Water there had been a spill of slewage sludge at the Tenterden sewage treatment works.

When an officer arrived concentrations of ammonia in the sludge at the Newmill Channel were so high the treatment works couldn't cope.

Samples taken showed effluent discharged from the treatment works was nearly eight times the fatal strength for fish.

Fortunately few fish were killed as a result.

Investigations found a piece of plastic pipework had burst in the sludge recirculation pump.

There had been a blockage the previous day and a valve was shut while repairs were being carried out by Southern Water.

Following this maintenance the valve was left closed and this caused pressure to build up and resulted in the pipe bursting.

Ben Tragett, Senior Environment Officer for the Environment Agency, said: "It is disappointing that Southern Water failed to solve the problems at the sewage treatment works during routine maintenance.

"As a result a totally avoidable pollution of an environmentally sensitive area occurred and there should have been contingency plans in place by Southern Water to deal with such an incident."

Southern Water apologised for the incident and paid for the clean-up operation as a result of the pollution.