Auckland could gain a new $3 billion-plus Waitemata Harbour crossing - tunnels or a longer bridge - within 15 years if Transport Minister Steven Joyce's programme is followed.

He raised that possibility yesterday after the Transport Agency confirmed engineering advice that the existing bridge's two-lane northbound clip-on may have only 10 to 20 years left before restrictions on heavy vehicles and possibly temperature controls are needed.

That is despite a cost blow-out also revealed by the agency yesterday - from $45 million to $86 million - for a structural upgrade of the two bridge clip-ons, due to be finished by the end of next year.

Contractors are installing 920 tonnes of extra steel in the clip-ons, up from earlier estimates of 313 tonnes, 600 tonnes and then 760 tonnes as design work evolved from when the former Transit NZ admitted in 2007 a slender chance of "catastrophic failure" without urgent repairs.

The Transport Agency is blaming the blow-out on increases in material and labour costs and difficulties gaining access to parts of the clip-ons.

It says the latest estimate will not be exceeded.

Mr Joyce this morning told Radio New Zealand said he wasn't too worried about the near-doubling of the cost of the strengthening work.

"To be fair to the agency, it's one of those projects where it's not until you're actually on the job and stripping back and looking at the steel work required and adding to that that you get the full extent of it," he said.

"Having said that, I would have liked to have heard about it earlier."

With traffic and temperature management measures such as hosing down sensitive parts of the bridge in summer, the agency still hopes to get up to 40 more years from the giant hollow box girder steel constructions added to the bridge in 1969.

But Mr Joyce told the Herald he believed, in view of the gloomy prognosis for unrestricted use of the bridge, "you would want to start looking at bringing the new one [harbour crossing] on by around the 15-year mark".

That would mean starting to build an alternative crossing "at the 10- to 11-year mark", he said.

"What is brought into sharp relief is that once the western ring route is complete, this third harbour crossing is likely to be the next most important thing for Auckland because it is such an important connection."

Although the new engineering advice confirms previous estimates for the northbound clip-on, it had some brighter news in disclosing what senior agency official Tommy Parker said was "significant capacity left" in its city-bound twin, which carries more empty trucks towards the port.

The advice has prompted campaigners to propose a combined user-pays walking and cycling path, to be tucked under the city-bound clip-on.

Mr Joyce also raised a possibility that the next harbour crossing may be a bridge, rather than the set of road and rail tunnels selected as the preferred option by the Transport Agency's predecessor and Auckland councils.

"I'm not knocking the tunnels option, but think the Government, given the size of the investment, would want to be convinced as to what was the best approach," he said.

A $1.3 million study last year identified a 3.9km tunnel route between Esmonde Rd on the Northern Motorway at Takapuna and Spaghetti Junction for $3.7 billion to $4.1 billion.

The study said a new bridge would be about $600 million cheaper, but it ruled that out because of the visual effect on the harbour and waterfront areas such as St Marys Bay and Westhaven.

But yesterday's developments have given new hope to promoters of an "Anzac Bridge" across the harbour on a similar alignment to the tunnels route, making it at least one and a half times longer than the existing bridge.

A group led by former Auckland City Council member Richard Simpson and including companies such as Jasmax and NZ Steel wants construction to start by 2015 to commemorate the centenary of Anzac Day.

It believes a new bridge could be built for $2 billion to $3 billion and that demolishing the existing bridge would release about 350,000sqm of land worth about $1 billion.

The Transport Agency wants to retain the existing bridge for local traffic, buses, cyclists and pedestrians.

Mr Parker also said the agency applied to the Auckland and North Shore city councils yesterday to designate a tunnel route through subterranean land titles on each side of the harbour.

That was needed to protect the route during the development of the Wynyard Quarter.

He told Radio New Zealand building a new bridge instead of tunnels had not been ruled out but it made sense to protect the route now for the tunnels, NZTA's preferred option.

Mr Joyce told the broadcaster he didn't regard the work as immediately urgent but the restricted life of the current bridge made the route protection prudent.

"Too often in this business you end up with corridors that aren't reserved - we're dealing with that at Waterview (for the final piece of the western ring route) at the moment - so it's good that they've moved ahead to do that."

- With NZPA