Featherston and Greytown have an extra two days of water supply, thanks to the recent commissioning of the Waiohone Treated Water Reservoir.
The reservoir, which was brought into service last month adds an extra 8 million litres to the existing water storage capacity.
It increases water storage for Featherston and Greytown by about five times.
The commissioning of the reservoir brings an end to the project which began in 2019.
Total project costs are yet to be confirmed, but the overall project includes the treated water storage bladder along with additional chlorination equipment at the outlet of the reservoir, and changes made to the treatment plant to operate with the new system.
A Wellington Water spokesperson said the project team was working to close out any commissioning issues there may be.
"Once this is completed, total project costs will be confirmed."
The new storage provides between two and two-and-a-half days of water for both Featherston and Greytown, depending on the time of year, they said.
Wellington Water chief advisor drinking water Laurence Edwards said the reservoir would offer much-needed water supply resilience to the community.
"The new treated water reservoir will provide a greater level of water supply resilience for both Featherston and Greytown, particularly in the summer months when demand is high," he said.
"This new asset provides buffer storage to help us better manage through adverse conditions or unexpected events, like the ones experienced in the last few years from storms and power cuts.
"It will also allow the treatment plant to be taken offline for an extended period of time when required, without impacting the delivery of safe drinking water to the community."
South Wairarapa District Council group manager of partnerships and operations Stefan Corbett said the commissioning of the reservoir was a significant step forward.
"This is a momentous achievement for all involved and will ensure a more reliable water supply," he said.
"I know our community has been looking forward to it and will welcome this significant progress."
The new treated water reservoir would provide a greater level of water supply resilience for both Featherston and Greytown.
The huge increase in treated water storage is particularly timely for Featherston residents, following a boil water notice that was in force a couple of weeks ago.
The reservoir was brought into service on 9 February.
But in the weekend after this, ex-cyclone Dovi wreaked havoc on water networks in Wairarapa and surface flooding contaminated Featherston's treated water.
The Wellington Water team was still working to prepare the reservoir to flush the Featherston water network of contamination on 18 February.
"To bring the reservoir online ahead of schedule, we have needed to introduce new processes and systems, and have had to do this carefully," Wellington Water said at the time.
Wellington Water finished flushing the network on 22 February and the boil water notice was lifted last week.
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