The plant, which was opened by the Duke of Edinburgh in 2010, is the only one in the UK designed to turn salty seawater into fresh water.
It was originally marketed as being able to offer an additional 150 million litres of water a day – but this has now been revised down by a third to 100 million.
It said that regardless of the plant’s current operational status, short term hosepipe bans would still be required to curb water usage.