Five million litres of sewage leaked into Botany Bay after a power outage on Friday, Sydney Water has revealed.

The leak occurred when a high-voltage powerline malfunctioned, causing Cronulla's waste-water pumping station to lose power and discharge sewage into the water at the southern end of Woolooware Bay.

Walkers at Brighton Le Sands on Botany Bay. Credit:James Alcock

As well as polluting Botany Bay, waterways in Georges River, as far upstream as Lugarno, Como and Oatley, are also affected.

The leak has angered environmental groups, with some calling for Sydney Water to have their licence reviewed.