The pipe supplying drinking water to Featherston lies exposed and leaking in a riverbed.
Councillors have now been asked to approve up to $600,000 of unbudgeted funds to repair and reinforce it urgently.
If the works are not done in time and the pipe fails, Featherston residents will be without water, and South Wairarapa ratepayers could foot a bill of between $850,000 and $6.5 million for an emergency contingency plan to truck water in, depending on the nature and length of the outage and river conditions.
In March last year, concrete casing came away from the pipe where it crosses the Tauherenikau River.
Following an assessment by Wellington Water [WW], the risk of the pipe failing was categorised as low, but its replacement was categorised as a "high priority".
In a recent report to South Wairarapa District Council's assets and services committee, it is now stated that there is a "high risk" that the pipe will fail, and therefore there is a priority on remedial works being done as soon as possible.
This comes after a leak was found in the pipe on 3 December.
The pipe has been monitored daily since then and repair work is expected to begin as soon as practicable.
Should these works not be completed, there is a near certainty of asset failure, the report states.
This would require the council to implement an emergency response plan in which water would be trucked in from Greytown.
"Whilst the response ensures a minimal level of service is delivered and public health is protected, it will result in an operational expenditure of anywhere between $850,000 to $6,500,000 depending on the nature and length of the outage, and river conditions," the council report said.
"Staff recommend immediate remedial actions to mitigate the risks associated with failure on the pipeline by undertaking repair on the existing leak and protecting the pipe in the short term and while summer weather conditions permit, and replacement of the vulnerable pipeline in the longer term."
If the works are approved, a leaking joint in the pipe would be repaired and the pipe would undergo substantial rock and concrete protection.
This would cost up to $600,000.
It is also recommended the council replaces the existing pipeline in the longer term to minimise the ongoing risk of washout.
There is no allowance for any works on the pipeline in the current Long-Term Plan, the report said.
A table of costs showed total works would range between $2.2m and $7.3m if there is no pipeline failure during construction.
The Featherston scheme was initially installed in 1965 and serves a population of 2600 residents with 36km of water main.
The network serves residential, commercial, industrial, and community users in Featherston.
Installed in 1998, the pipe supplying water to Featherston was encased in concrete and buried beneath the river channel.
The riverbed has since moved and degraded to a point that the previously buried pipe is now sitting exposed in the riverbed.
The likely mode of failure for the river crossing pipeline is debris impact during a storm event, where the river level is expected to be high.
The section of pipe that is exposed is about 23 metres long.
Of the exposed section, about 15m of pipe is concrete encased and the remaining 8m is made up of steel pipe.
The leak only became visible when the river level was low, the report said.
The area of the pipe around the leak suggests it may have been leaking undetected for some time and was not visible due to river levels.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air