An $8 million bulk water duplication pipeline project to secure supplies to greater Hobart has commenced.


Southern Water says the West Derwent Pipeline, constructed in 1962, brings water from the Bryn Water treatment plant to Hobart's eastern and western shores.


The 3.2 kilometre section to be duplicated runs from Berriedale to near the Elwick racecourse then crosses over the Bowen Bridge.


Southern Water chief executive, Mike Paine, says it is the only section of the bulk supply that does not have a "back-up" pipeline and will address the risks inherent in an ageing pipeline including corrosion.


The bulk water system delivers up to 70 million litres of water a day.