Auckland's ageing Harbour Bridge can't take any more strengthening and traffic will need to be restricted in order to maintain its "structural integrity".

Waka Kotahi says the bridge has previously been strengthened several times but it is not possible to do it again due to the weight of the steel that would need to be added.

Instead, "active traffic management" will need to be introduced.

It is costing "millions of dollars a day" as traffic is diverted because of strong winds on the bridge or roading needing to be fixed, Auckland's Chamber of Commerce chief executive Michael Barnett told Newstalk ZB this morning.

"For Auckland, we seem to wait until there's a problem and look to fix it instead of being strategic and looking ahead, knowing we are growing at 50,000 people a year, knowing we have to do the infrastructure and just talking about it instead of doing something," he said.

A November briefing paper to Transport Minister Michael Wood warned the "loading restrictions" would be needed within the next 20 years — but Waka Kotahi general transport service manager Brett Gliddon hints it may actually be much sooner.

Gliddon said active management of traffic on the bridge would not be needed within the next 12 to 18 months but did not elaborate further on a timeframe.

Regardless of the timeframe, talk of restrictions has renewed calls for an alternative harbour crossing to be prioritised to prevent the city from coming to a standstill in the future.

The revelations came at the annual review of Waka Kotahi New Zealand Transport Agency at a parliamentary select committee yesterday.

After a question from National MP Christopher Luxon, Gliddon said it was no longer possible to strengthen the bridge, which is crossed by 170,000 vehicles on weekdays.

"We believe we've strengthened it as much as we possibly can and we can't add more steel into it ... it's counter-productive."

He said maintaining the "structural integrity of the bridge" could involve restricting heavy vehicles, limiting the lanes they could use, the number of heavy vehicles on the bridge at one time, or the time of day they cross.

Road Transport Forum chief executive Nick Leggett said the fact remediation work was no longer an option was concerning given the lack of alternative options and there needed to be urgent plans for another crossing.

"Auckland is a key arterial route through our biggest city and our biggest economy.

"We don't accept that there isn't a budget for this, and there needs to be priority given. Any sort of build needs to start in the next two or three years."

AA spokesman Barney Irvine said the briefing paper highlighted how vulnerable the bridge was and emphasised "the importance of the whole discussion around an additional harbour crossing".

"That's a project we cannot allow to drift."

The bridge was last upgraded in 2010, during which approximately 900 tonnes of steel was bolted onto the clip-ons to extend the bridge's life for between 20 and 40 years.

Luxon said he was frustrated that the bridge was nearing the end of its useful life without plans in place for another harbour crossing.

"There's no future-proofing or commitment to a tunnel, or second crossing yet. We don't know if it's going to be a tunnel or a bridge or anything at this point.

"We've got a world-class city and in that city, a guy drives his truck into a bridge and everyone's out of action and inconvenienced for two weeks."

A truck crash on the bridge during strong winds in September last year caused structural damage. Photo / Supplied

Auckland came to a standstill last year — and people were asked to work from home — after two trucks were blown into the bridge in wind gusts of up to 127km/h, damaging its structural integrity.

National Transport spokesman Mike Woodhouse said he hoped the Government did not wait for restrictions on the bridge to plan for another crossing.

"We've been talking about a second harbour crossing for I don't know how long. We need to get some clear stakes in the ground."

He said the briefing document, released to the National Party through the OIA, should have been a wake-up call to the agency and the Government.

"What we'd known is that the traffic pressures were demanding the need for better harbour crossing access," he said.

"Now we know it's not just congestion pressure but lagging infrastructure."

Transport Minister Michael Wood referred questions about future restrictions to the bridge and the progress of another crossing back to Waka Kotahi.

Waka Kotahi Chair Sir Brian Roche said the agency still needed to determine the form of public transport that would be used on a second harbour crossing, which has been estimated to cost several billion dollars.

He said the engineering challenge was immense and would require overseas expertise.