Council development engineer Peter Morton told the Vincent Community Board yesterday he did not have cost estimates, but a 2007 study produced some ranging between $1.77 million and $13.62 million.
For more than 35 years, the water supply has been sourced from six bores on the true left of the Clutha River but under national drinking water standards, it has an E rating - the worst available - and has problems with water hardness, scaling and occasional taste and odour issues.
The borefield is also next to a closed landfill, though there is no evidence to suggest there is any contamination.
In a report to the board, Mr Morton said a 2011 study found redesigning the bore site was unlikely to remove the scaling tendency. A 2012 study investigating the possibility of a new bore site about 1.5km northwest of the present site found that site, known as Ngapara Pools, was likely to have similar scale-forming properties.
He told the board the council had "reached a fairly critical point in this investigation" and the only "realistic" options were now to source water from the Clutha River near existing Alexandra water infrastructure, although turbidity would be an expensive but solvable problem, or a lake or groundwater supply near the Clyde supply bore.
He said in order to obtain a good estimate of the cost of options, engineering investigations would be needed, at added expense. In the 2012-13 year, $200,000 has been budgeted for further research.
