Being hit by a 40kg cladding panel that plunged 13 storeys from an Auckland apartment building this afternoon would be like being crushed against a wall by a small car - if you were lucky.

That's according to University of Auckland physics lecturer Dr Geoff Wilmott, who made some rough calculations for the Herald after wind ripped the panel from the Victopia building on the corner of Victoria St West and Nelson St.

Police have closed a section of Victoria St due to debris and out of concern more panels could come down.

Traffic is at a standstill in the area, with a 45-minute delay to get out of the Skycity carpark on Nelson St.

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A witness said she heard a "very loud bang" then saw dust fly up from behind a construction fence when the panel fell just after 1pm.

Several pedestrians on the footpath below had visibly jumped at the impact before retreating to a covered tunnel.

Pedestrians visibly jumped when a cladding panel plunged 13 storeys to the street nearby. Photo / Will Trafford
Pedestrians visibly jumped when a cladding panel plunged 13 storeys to the street nearby. Photo / Will Trafford

It appeared the cladding had not fallen straight down but had been caught by the wind and pushed several metres sideways as it fell, she said.

Dr Wilmott said assuming the panel measured 1m x 1m, and it fell edge-on, it could have reached speeds of around 100km/h by the time it hit the ground.

Anyone standing underneath would have sustained serious injuries even if it hit face-on - the best-case scenario, as that would spread the impact.

"The pressure would be about the same as for a small car (1000kg) running into a wall at about 20km/h," he told the Herald.

"Not too bad for the car but you don't want to be caught between it and the wall."

If it had hit edge-on, it could have impacted at 10 times that pressure - like a small piece of a car hitting you at 200km/h, he said.

"If you had the choice, you'd want to be hit face-on ... It's definitely going to cause serious injury if it hits someone."

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Police cordoned off an area around the building this afternoon and have since closed the road between Hobson St and Nelson St, causing traffic chaos.

Vehicles are being detoured, with bus routes affected including the 22, 24, 75, 106, 125x, 129, 132, 133 and the InnerLink.

Police have cordoned off the intersection. Photo / Simon Gianoutsos
Police have cordoned off the intersection. Photo / Simon Gianoutsos

The incident came amid heavy rain across much of the North Island, with severe weather warnings in place from Northland through to Gisborne, Hawke's Bay and Taranaki.

The Auckland apartment building is subject to $40 million of remedial work which began in March. The panel that fell to the street was on the Victoria St West facade of the building.

Inspector Cornell Kluessien said traffic was being detoured as a precaution.

"The wind and the rain is the concern because now that panel is missing more water will get behind it. Between the wind and the rain another panel may get loose and blow onto the road."

Fire and Emergency NZ northern communications shift manager Murray Bannister said as of 3.30pm there had been no other weather-related callouts.

Aucklanders woke up to a deluge today, with surface flooding quickly appearing on some motorways.

Surface flooding is affecting several roads around the North Island, with at least one closure.

MetService has issued severe weather warnings for Auckland, Northland, Waikato and the Bay of Plenty, with heavy rain predicted to cause surface flooding, as well as potential slips and a rapid rise of water levels in streams and rivers.

Taupō, northern Hawke's Bay and northern Taranaki were also warned of heavy rain today.

Motorists are advised to avoid State Highway 12 near Opononi in Northland due to flooding since midday.

And surface flooding on SH2 in Tauranga has been causing issues since this morning. Caution is advised between Elizabeth St and Waihi Rd.

Urgent repairs are also required on Saddle Rd near Palmerston North due to the wet weather.

Flooding has intermittently blocked a section of Auckland's Northern motorway this afternoon, with northbound traffic forced to merge to avoid the huge puddle just after Upper Harbour Highway, heading north.

The NZ Transport Agency warned drivers to take extra care and expect delays. The right-hand lane has twice been closed and reopened.