The main will be shut down tomorrow (Saturday 13 August), meaning businesses and residents on the south side of Manners Street between Cuba and Victoria streets will be without water.
Wellington City Council's Acting Infrastructure Director, Mike Mendonca, says businesses and residents will be notified in advance but this may be at short notice.
Motorists and bus commuters should also expect delays, he says.
"Unfortunately we don't know exactly how long the water will be shut off," Mr Mendonca says.
"That will depend on how quickly we can excavate and expose the pipe work, and the nature of the repairs.
"The job is complicated because of access to the main. Rather than break through the reinforced concrete road, the plan is to dig a hole in the southern footpath, and then tunnel under the concrete road foundations."
Businesses have been given written notice of the works and have been advised to store water to carry them through the shutdown period.
It is expected to take two days to excavate and repair the main, followed by another two days to reinstate the footpath and the road.
"It is possible other buried services and conditions could mean that work will stretch into next week," Mr Mendonca said.
"People will undoubtedly be inconvenienced and all we ask is that they be patient. The reality is that infrastructure needs to be maintained and sometimes repaired."