Auckland Council has been slow to improve the city's stormwater infrastructure, which a 2011 council report said needed $9.9 billion to fix over the next 50 years.
The bill comprises $5.4b to bring the stormwater system up to scratch and $4.5b to cope with growth.
It took four years for the council to match the $105 million the former councils spent before the Super City was formed in 2010.
In six years, the council has spent $640 million on growth and renewals for stormwater - half of the $1.2b spent on IT.
The revelations about stormwater spending follows Mayor Phil Goff's call to investigate the failure of a New Lynn culvert after Sunday's rain bomb and the struggling Ardmore water treatment plant following the destructive downpours.
"You've got to find out what's happened. What's caused it? Could it have been prevented? Coould it have been mitigated against?" Goff said during a visit to New Lynn late yesterday.
Council healthy waters general manager Craig McIlroy told the Herald in January that the current and planned level of capital investment was appropriate.
"Indeed the Healthy Water's Asset Management System has recently been internationally benchmarked as representing global best practice," he said.
Capital spending on stormwater since the formation of Auckland Council.
2010/11(part year) $40m
2016/17 (forecast) $150m
In response to a Herald investigation into sewage overflows into the Waitemata Harbour almost every time it rains, Goff asked for work to be done by June 30 on bringing forward work to ensure clean and safe beaches, despite the cost to ratepayers.
Watercare chief executive Raveen Jaduram has said the council is allowing development knowing there is no adequate stormwater system and that this will result in more harbour spills.
In an email to Auckland councillor Mike Lee last year, Jaduram was critical of the council's approach to stormwater. He said stormwater flowing into the combined wastewater and stormwater system immediately significantly increased the flow.
Putting detention tanks into new developments to slow down the stormwater flow when it rains "is not a sustainable solution", the Watercare boss said.
McIlroy said future growth would not have a significant effect on water quality.
"The adverse effect of more housing will be offset by short-term improvements such as requiring onsite detention of stormwater in combined areas before the new interceptors are built," McIlroy said in January.
Watercare is spending $1.7b to take stormwater and wastewater to the Mangere and Rosedale water treatment plants and reduce wet weather overflows.