Auckland mayor Wayne Brown says he promised to "sink" the previous Labour government's plan for a tunnel beneath the Waitematā Harbour.

The mayor has revealed a new idea for bridging the gap between the North Shore and the central city - with a second harbour crossing that would not require a tunnel.

He had proposed a causeway or series of bridges from Meola Reef in the central suburb of Point Chevalier to Kauri Point in Birkenhead, on the North Shore.

Labour's plan for a second harbour crossing - which included rail and road tunnels - was estimated to cost more than $45 billion.

The current government's focus was on an additional crossing for vehicles.

Brown told Checkpoint host Lisa Owen the government had told local councils to "stop wasting money - and so should they".

"There's no way that ... New Zealand can afford [a tunnel]. The tunnel solution is anywhere between $34 and $55 billion which eats up all the road budget for 10 years - now that's just untenable."

Meola Reef already jutted out "80 percent" across the Waitematā Harbour, so it made sense to build a second crossing there, he said.

"Our forebears weren't stupid - they built the bridge in the shortest gap, which is Northcote Point to Westhaven. And the next shortest gap is to the end of the Meola Reef."

Owen asked Brown whether the land around the reef was stable, suggesting it was a "bog hole of reclaimed land".

"The Meola Reef is a natural reef... it exposes itself at low tide so there's no boats going through, and makes it easy to build out [into the harbour].

"Bridges are a lot cheaper than tunnels anyhow, and short bridges are even cheaper."

Brown said the new bridge would go to a "big empty area" on the Shore owned by the government, which could subdivide it to fund the second harbour crossing, "and the profits would almost pay for the bridge".

"So instead of something which is completely unaffordable, we'd have something which is quite quick and available, and relatively cheap."

Owen pressed Brown on how much his proposed second crossing would cost.

"About 10 percent or less of the other. It's hugely cheaper; massively cheaper. [Unlike tunnels], bridges don't require air conditioning, they don't require air flow... there's nothing wrong with the existing bridge."

Owen also asked Brown what sort of time frame they were looking at.

"In New Zealand, it takes longer to talk about things than it does to build them," he said, without providing specific details.

Brown compared Auckland to river cities like London and Paris.

"When the first bridge got full, they didn't build another one immediately behind it, or a tunnel under the [river] - they went along river for a bit and put another bridge in, so you get a different group of people travelling to different places, so you reduce your traffic demand...

"The net cost could be very low, when you consider that the land where you suddenly upvalue on the North Shore was owned by the government anyhow.

"They want to densify the population in Auckland and I'd rather have them in the North Shore in empty land rather than building over our vegetable areas at Pukekohe, which is completely stupid."

Auckland Mayor Wayne Brown. Photo: RNZ / Marika Khabazi

Owen again pressed Brown for an estimate on how long such a bridge would take to build.

"I've just had an international construction company come out... they glanced at what I was doing, rushed back and said, 'We'd love to get on with this.'

"So it'd take a helluva lot less time than it'll take the government to buggerise around, making a decision about it."

The proposed tunnel under the harbour would "never exist, I can assure you that", Brown said.

Owen then asked, where the roads and motorways linked to the new crossing would go?

"Well, there's plenty of traffic going towards the Point Chevalier area anyhow, coming off that western motorway [SH16] ... so that's a good idea, it stops that traffic fighting its way through the city to get to the Harbour Bridge now."

Once over the harbour, the crossing would go to "a whole lot of new suburbs", he said.

"For a while, for just the people over there it would make it worthwhile."

Any subsequent motorways would "slowly appear", he added.

Residents in Point Chevalier and surrounds were already dealing with Meola Road being ripped up and narrowed to a single lane on each side, said Owen.

What would the proposed crossing and linking roads do for this area?

"Hang on. You're blaming me for stuff for me that AT does - and I don't have control of AT yet... but that's coming.

"You don't have to rip up a road to make it work ... grown-up people don't do that."

So all the second bridge traffic would go up Meola Road? she asked.

"I think you're being a bit negative here... it's a helluva lot better than a tunnel... a tunnel goes from an existing traffic jam to an existing traffic jam and the bridge itself is not crowded, so why on earth would you build it in the same place?"

Brown said he had not yet spoken to Minister of Transport Simeon Brown about the idea.

"I did tell the prime minister I was going to sink the bloody tunnel for him, and save him a helluva lot of money. Because it's embarrassing for a prime minister to be saying we're going to be spending the entire road budget of the next 10 years on one project, in one sitting."

Owen asked if Prime Minister Christopher Luxon was pleased with this exchange.

"I don't know, he probably is, I suppose, I hope he will be."