Phil Goff says his rival's plans for Watercare will cost Auckland households up to $300 a year.
The Auckland Mayor faced off against John Tamihere in a fiery debate on Newshub Nation on Saturday morning.
The two former Labour MPs are the likely frontrunners for the mayoralty of the country's biggest city, which will be decided in October.
Tamihere wants to partially privatise Watercare, the council-controlled organisation that supplies Auckland's water. He says selling 49 percent of the company will "release good value" while keeping it under the city's control.
"That will push the water rates up of every Auckland household by $200 to $300 a week - I've done that calculation," Goff told Newshub Nation host Simon Shepherd.
"What's more, the people who are going to be hurt by that are first of all the large families - when you've got kids you use more water, and they're the families already struggling with mortgage and rent - and it's going to hurt most of all low-income families, because the water rates are a higher percentage of their incomes. So that's his promise - pushing water rates up by $200 to $300 a year."
Goff says this is because a private investor will demand returns of 7 to 10 percent a year, while the council does not require Watercare to pay dividends.
Tamihere admitted he didn't know how much water costs would go up.
"Water rates will go up, but not to the extent he says. Because what'll happen is, what I'm endeavouring to do by rebuilding the balance sheet in the city is smooth the pain, rather than rocket the rates all the time and the stealth taxes."
Tamihere accused Goff of being no stranger to privatisations, having been a part of the Labour Government which sold Air New Zealand in the late 1980s. Goff said that was a "nonsense" argument, since he was also a part of the Labour Government that bought it back in the early 2000s.
Tamihere also took aim at Goff's record on rates. Goff has kept the average rate rise in his first term to 2.5 percent, but there have been other charges dumped on Aucklanders in that time, including the regional fuel tax and targeted rates for cleaning up the city.
"It's death by 100 cuts with this guy. It's not just rates - what about the 11.5 percent [sic] Goff gas tax? ... what about water going up? What about refuse going up? There's a whole bunch of stealth taxes that this guy uses, then gets you to focus on the [rates]."
Goff defended the rates aimed at environmental clean-up, saying Auckland's beaches and harbours will only take nine years to clean up, as opposed to 30 under the old plan. He even called it his biggest achievement as Mayor to date.
Goff also defended the regional fuel tax, pointing out it came with an $18 billion injection of capital over 10 years from central Government to spend on infrastructure. Tamihere reckons he could get more.
"I don't want discrimination on Aucklanders only," said Tamihere, promising to repeal it. "No other city and no other region pays it. He signed us up to it."
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Tamihere's running mate Christine Fletcher ironically voted in favour of the 11.5c a litre tax.
Goff said Tamihere's strategy of "going down [to Wellington] and banging your fist on the desk and demanding things [won't work]".
The elections are in October.