Demand for water in Wellington could outstrip supply in as little as six years, prompting authorities to undertake a business case into household water meters across the metropolitan region.
The supply-demand balance for water was last assessed in 2017, but a report tabled at a Wellington Water Committee meeting this year shows the situation has changed.
The population growth rate has exceeded those earlier forecasts, meaning the region will need to find a new water source well before 2040.
It could be needed as early as 2026 if demand and projected population growth continue on the trajectory they're on now, a Sustainable Water Supply Target and Policy report said.
Water metering has proven controversial in the region over recent years, with politicians kicking the can down the road or voicing concerns over any system that involved charging people for water.
But Auckland's water crisis is a dire example of a water supply gone wrong and why the debate in Wellington can no longer be pushed to the back of the queue.
An initial 10 per cent reduction in demand would be required over the next six years in the capital to offset growth over the same time period.
That's the same as a reduction of 40 litres per person per day in gross demand.
Alternatively, a new source and associated network capacity increases would cost more than $250 million.
Meanwhile the Te Marua Water Treatment Plant also needs upgrading to the tune of $20 million.
It was built between 1980 and 1987 to treat water taken from the Hutt River at Kaitoke.
Wellington's water supply network should meet normal demand up until a drought is more severe than a 1-in-50-year event.
But this level of service hasn't been met since 2017.
Constraints at the treatment plant mean Wellington Water would be unable to meet normal demand from about a 1-in-15 year event.
The upgrade, including the 10 per cent reduction in demand, will need to be completed in the next six years to defer any investment in a new water source.
The Wellington Water report said the level of leakage and water demand relative to other cities suggested those savings could be achieved.
"However, with limited information about where and how water is being used, the need for customer behaviour change that we can only attempt to influence and cannot control, and the relatively short timeframe, means that achieving the targeted savings will be challenging."
A move to universal metering could reduce residential water demand by up to 20 per cent, mainly through identifying leaks in supply pipes and fittings, the report said
The investigation into water meters is currently being finalised and expected to be tabled at a Wellington Water Committee meeting in September.
The business case, undertaken by Wellington Water and commissioned by Greater Wellington Regional Council, is focused on identifying leaks and influencing consumer behaviour.
A 2018 Wellington Water customer engagement survey showed 70 per cent of respondents thought conservation was necessary and 65 per cent thought they should be conserving more water.
Greater Wellington Regional Council Chair Daran Ponter said he expected water metering to have a big effect on demand because people would be more conscious about the resource they're using.
He said any decision would ensure low-income families would have an adequate minimum supply of water before a charge was levied.
The business case for water meters covers the areas of Hutt, Porirua, Upper Hutt and Wellington city councils. Kapiti Coast and South Wairarapa councils already have them.
"If water metering is to have a chance in the Wellington region, beyond those two territorial authorities, then we need to take a regional approach", Ponter said.
"If we don't take action to collectively limit the amount of water we take as a region, then ratepayers will end up having to build more dams to hold our storage water- that's tens and tens of millions of dollars."