That's the maximum that would be needed, with spending starting lower and increasing in later years.
Wellington Water chief Tonia Haskell told Morning Report it was up to Wellington City Council to decide how much it could actually afford to spend.
One councillor has suggested the council could cut staff to free up funds, while the mayor said she was open to putting other plans on hold to find the money.
Wellington Water had said it would cost $1b a year for 10 years to fix the region's water woes.
But Haskell said there was a limit on what could be delivered.
She said it was restrained by contractor availability, staff capacity, and an acceptable level of disruption in the city.
"We have to dig up roads, we have to interrupt private properties, there's a maximum amount that the city would be able to sustain."
The $2.5b was how much Wellington Water believed it could deliver in the city over the next 10 years, Haskell said.
"You have to build capability ... it does take awhile to do that, as I say we have been doing that steadily for the past few years."
A review into the efficiency of the agency is due out later this week.
Despite some reports it will be critical of Wellington Water, Haskell said she welcomed its release.
"We can always learn and do things better.
"There does need to be investment in our systems and our processes and people, which is also lacking, to kick us up a notch."
Upper Hutt City Council wants assurance
Upper Hutt Mayor, Wayne Guppy, told Morning Report he had "major concerns" about Wellington Water's ability to get things done.
He said his council had been asked for more money, but wanted an assurance the work would happen.
"Over the last 5 years, we've had more than about $18m of carry-overs, unspent by Wellington Water, so the history is not great."
He acknowledged capacity was an issue, but didn't think disruption should be a factor.
"People want things fixed."
Residents would rather traffic disruption than putting up with water restrictions while watching it flow down the gutter, he said.
Guppy met Local Government Minister Simeon Brown on Monday in what he described as a "straight forward" meeting.
He said they discussed the issues facing Wellington and the county, and possible models for change.
"Anyone in the country that thinks Government is going to turn up with a cheque book and pay the bills, that is not going to happen."
Difficult decisions ahead
Former Wellington mayor Dame Kerry Prendegast told Morning Report councillors faced some difficult decisions.
"Local government is absolutely about potable water and sewerage, that is the absolute thing you have to do when you're elected."
But Prendegast, who has been a champion of the Town Hall earthquake strengthening, said it should be able to continue.
It would be more expensive to halt the restoration than to complete it, she said of the much delayed and over-budget project.
Despite Mayor Tory Whanau saying all options were on the table, Prendegast said the Town Hall was a significant asset and its strengthening should be finished despite the water crisis.
Other "vanity projects", such as the cycleways, could be put on hold, she said.
RNZ
