Councillors have delivered a mixed verdict on the possibility Aucklanders could be levied another tax — nicknamed a "toilet tax" by opponents.

The Weekend Herald revealed today Auckland Council and the Government are considering the tax to pay for a $1 billion sewer tunnel running deep below our biggest city and aimed at allowing for future growth and significantly reducing sewage and stormwater overflows into harbours.

But the potential tax also comes hard on the heels of a 11.5c a litre regional petrol tax, introduced six days ago, and a bed tax for accommodation providers.

Albany councillor John Watson said Aucklanders were being hit with a "double whammy" of new charges and asset sales — both over a short period and despite promises amalgamation would cut costs. He wanted an independent review of the Super City.

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"When the Super City was set up the fundamental selling point to Aucklanders was that it would result in reduced costs. That hasn't eventuated because we are now getting additional charges for services you'd expect to be included ... in rates."

Waitematā and Gulf councillor Mike Lee said too much money had been wasted on an IT budget blowout and a new council headquarters.

"Watercare did the massive upgrade of Project Manukau, the Waikato pipeline and Project Hobson, and they were all on the books about the same time, but it was easily affordable to take on that debt. Not anymore."

Rodney councillor Greg Sayers said Mayor Phil Goff hadn't lived up to his campaign pledge to find 3 to 6 per cent savings from the council budget.

"This would amount to between $70 million and $140m ... enough to pay for the new wastewater infrastructure and meaning Aucklanders would not have to pay Phil Goff's toilet tax."

However, other councillors took a different view.

Waitākere councillor Linda Cooper said a workshop had been held to discuss potential ways to pay for the sewer tunnel project.

"I was quite alarmed to see [today's story]. I'm getting really tired of this fake news coming through all the time."

However it was paid for, the project — and other long overdue infrastructure upgrades — were needed.

"It's not a wish list, it's a needs list. I would think ratepayers would want us to workshop ... to find the best way to deliver this."

Papers released under the Official Information Act outlined a novel way of paying for the sewer tunnel and previous scenarios for a partial sale of council-owned Watercare.

An alternative way of funding infrastructure, known as a special purpose vehicle, is being worked on by the council, Watercare, consultants and the Treasury as observers.

Deputy mayor Bill Cashmore said that would be a way of paying for the infrastructure, which was a century overdue.

The option shouldn't be dismissed just because it was something new.

"I'm in favour of investigating [and seeing] what's the best outcome."

Manukau councillor Alf Filipaina said a decision was yet to be made — and any decision would be prefaced by public consultation — so he didn't understand why fellow South Auckland councillor Daniel Newman had gone public.

"I don't know what he's up to."

Other councillors couldn't be contacted.

The tunnel project would be owned by a Crown entity and paid for through a fixed charge collected by the council over 25 to 40 years.

But both Newman and National MP Judith Collins have criticised the charge, calling it a "toilet tax".

"I fear the proposed SPV will likely be judged as a form of privatisation to repay debt holders that will not win public support," Newman wrote in a letter to Goff and councillors.

Goff told the Weekend Herald the council could not afford to borrow more because it was right up against a debt ceiling that, if breached, would lead to a credit rating downgrade and higher borrowing costs.

The council had workshopped several options on an SPV for the project, none of which had been recommended or decided yet, he said.