The government is seeking advice on whether to bring in a toll on the existing Auckland Harbour Bridge, to help pay for a second Waitematā Harbour crossing.

The Infrastructure Commission has suggested a toll as high as $9, a figure the Transport Minister says would be a "big decision" to make.

The government continues to mull over its options as to what a new crossing would look like, such as a tunnel or second bridge.

In the newly-released National Infrastructure Plan, the Infrastructure Commission said new revenue would be needed to fund the crossing.

High-level analysis suggested a $9 toll "on both new and existing crossings" could raise between $7 billion and $9 billion, depending on the tolling period.

"Higher tolls may not raise more revenue, as they would divert too many users and erode viability, and tolling only the new crossing would sharply limit revenue," the Commission wrote.

"Other funding mechanisms are possible, but would likely require non-users to contribute funding which may not be considered equitable or favourable."

When the bridge was first opened in 1959, motorists had to pay 2 shillings and 6 pence, a figure the Commission said equalled around $9 in 2025.

Tolls were removed in 1984.

Transport Minister Chris Bishop said the new crossing would be the biggest infrastructure project New Zealand has ever done.

While the new crossing would be tolled, a question remained over whether the existing bridge would be tolled as well.

"We are working our way through that. That's a very big decision for the country to make," he said.

Bishop said he would not get ahead of any decision, and the government was working through it in a "methodical and comprehensive way" as the Commission said it should do.

"We're working our way through quite a complicated series of funding questions and financing questions around the second harbour crossing. It will be a very large infrastructure project. All large infrastructure projects have to be paid for. So we're working our way through that."

He said "in theory," a new crossing should be able "wash its own face, financially," due to the number of vehicle movements.

ACT leader David Seymour, an Auckland-based MP, said $9 per trip added up to $90 a week for some people who would already be trying to pay "tough" bills.

"I think you're going to struggle with that level of price. But you could imagine that maybe at a peak hour, when it was mostly buses and ridesharing, maybe for a part of the day. But I don't think making everybody pay $9 with no alternative is going to fly."

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