A £13m upgrade by NI Water to improve the security of the water supply at Drumaroad Water Treatment Works in Castlewellan is now under way.
The work will continue until spring 2021.
Infrastructure Minister Nichola Mallon said the work "highlights the impact investment in our infrastructure can have".
She added: "This major plant treats water from Silent Valley reservoir which delivers around 140 million litres of water every day to over 200,000 homes in Co Down and Belfast.
"Once complete this will have a positive impact on the lives of up to 540,000 people - a quarter of the Northern Ireland population."