Risk of critical assets failing in the meantime has been described as high, but the council is confident increased spending on repairs and maintenance in the next nine years should reduce the threat.
Reprioritising capital expenditure could be needed after that.
However, such challenges may end up being handled by a council-owned water company, rather than council staff, if a groundswell of support emerges for this approach.
Commentary about the situation has been included in the council’s 2025-34 draft long-term plan consultation document and supporting documentation approved this week.
Some of the information was not part of public consultation material indicated to councillors last month — such as the $1b estimate — and was included with some prominence after input from auditors.
A 30-year infrastructure strategy described the backlog as a deficit in renewal expenditure and replacement of ageing infrastructure.
"The deficit is shared proportionately between water supply 32%, wastewater 44% and stormwater 24%."
The 2025-34 draft long-term plan consultation document had an expanded section about Three Waters.
"We have opted to develop a renewals budget across the 30-year infrastructure strategy to progressively complete these renewals so that there will be no backlog by 2055," it said.
"We have planned to address the backlog over this timeframe because we need a programme of works that is affordable to ratepayers."
The draft nine-year plan would reduce the backlog by about 4%.
"Beyond this period, a reprioritisation in capital expenditure will be required to address the remaining backlog [until 2055].
"Whilst the risk that critical assets may fail during this period may be high, we believe that this risk is mitigated, as we have significantly increased our provision for repairs and maintenance over the term of [the nine-year plan]."
Some assets were in better condition than expected.
"Renewals are prioritised, based on their condition and performance, and not just their age — fixing those assets that need it most."
Cr Sophie Barker said some previous councils had lacked focus on infrastructure and a catch-up would take a while.
"We can’t afford to do it all at once, and right now."
It was stated in the consultation document the first five years of work would focus on areas of highest risk.
"New capital investment across the nine years of the plan and beyond aims to improve water supply resilience, water use efficiency, and to upgrade wastewater and stormwater networks."
The council included $1b in its draft nine-year capital budget for Three Waters infrastructure.
Consultation about the nine-year plan and future water delivery services starts on Monday and runs through April.
