A downgraded wind warning has reduced the risk of the Auckland Harbour Bridge closing today.

The New Zealand Transport Agency earlier warned the bridge may close if wind levels became strong today.

Auckland Highway manager Brett Gliddon said closing the bridge would be unprecedented but if predicted winds of up to 160km/h barrelled through the city this afternoon, travel on the bridge would be too dangerous.

The MetService had since downgraded the wind warning saying gusts of 120km/h or were likely in Auckland.

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But the Transport Agency said it would monitor the bridge and if winds reached 120km/h restrictions were likely, even if it was not totally closed.

Restrictions could include lane closures, speed reductions and a ban on high-sided vehicles.

Gliddon warned any closure of the Harbour Bridge would have a dire knock-on effect across Auckland's motorway network on the eve of one of the busiest holiday travel weekends of the year.

"Closing the bridge is not a decision we will take lightly because it will have a significant impact on traffic flows throughout the city. However our first priority is safety and that's what will determine any closure," said Gliddon.

"If the bridge is closed to traffic this will have a flow-on effect across the rest of the motorway and local road network, so motorists should expect long delays, and avoid travelling if they don't need to."

Alternative routes between the city and North Shore would be via State Highways 16 and 18.

"We encourage people to think ahead and start planning alternative ways to get home or to their destinations later today and to consider delaying any non-essential travel across the bridge," he said.

Staff would monitor wind strength and direction throughout the day and watch the effect wind was having on vehicles travelling over the bridge.

Any closure would happen in stages. Speed reductions were likely across the bridge and the number of lanes open to traffic would also be cut.

Meanwhile high-sided vehicles and motorcyclists were asked to avoid using the bridge as the winds rose throughout the day.