A "mind-boggling" 80 percent of Featherston's treated water was lost in the network in October last year, a council report shows.
Following continued network repairs, just over 60 percent of the town's water is lost before it reaches the tap, as at December.
This is a far cry from nine percent water loss in Featherston in 2019, but monitoring methods have changed over the past few years.
It is not known how much of the town's water loss can be attributed to the exposed leaking water main crossing the Tauherenikau River, which was recently classed as having a high risk of failure in a severe weather event.
District-wide, 57 percent of treated water is lost in the South Wairarapa network, a recent council report shows.
South Wairarapa councillor Alistair Plimmer held Wellington Water's feet to the fire as the data was discussed at this week's Assets and Services Committee meeting.
"I read your report, and it seems it is the same answer every time; that we need to do more investigation," Plimmer said.
"I thought we had been doing a lot of investigation.
"The cost to ratepayers of these leaks is very astronomical because of the cost of treating water."
Wellington Water (WW) took over the council's water maintenance in 2019.
WW network management group manager Jeremy McKibbon said unmetered or unauthorised use may be to blame for the water loss.
There were some assumptions made in the data, he said.
"As more data and information come to hand, we can build them into the analysis and have greater confidence in the outputs."
He said the high amount of leakage was a symptom of "an ageing network in poor condition".
In his report to South Wairarapa District Council, McKibbon said WW network service crews had noticed a recent trend in water leaks appearing adjacent to previous repairs on water laterals, requiring full lateral replacement.
"This appears to be due to the degrading condition of lateral service pipes which make up approximately 50 percent of all water leak faults."
Councillor Garrick Emms labelled the water leak data "horrendous" for Featherston in particular.
"I just find it mind-boggling; we are trying to do something to fix the water pipeline across the Tauherenikau River [which supplies Featherston] and my colleagues in Featherston are losing 80 percent of their water.
"Please find an answer."
WW's annual report covering the 2019 year showed Greytown was suffering huge water losses back then with a whopping 39 percent of the ward's water not reaching residents' taps that year.
Greytown now has about 55 percent water loss, as at December 2021.
Martinborough, which had a water loss of 13 percent in 2019, is now losing about 46 percent.
Last year, the government decided to push ahead with Three Waters reforms, and take control of water services and assets from local councils, despite considerable opposition - including from South Wairarapa District Council.
The proposed reform of the country's three water services - drinking, waste, and storm water - has caused outcry across many local councils which currently have ownership of billions of dollars worth in water assets.
As part of the reforms, four new water entities would take on the water assets currently owned by councils.
A working group has been established, made up of local government, iwi, and water industry experts, to work on how the entities will be governed.
This working group is set to report back to the Local Government Minister by 28 February with its recommendations before the Water Services Entities Bill goes to Select Committee this year.
People can make submissions to the Bill when it is before the Select Committee.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air