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.
Meanwhile, two breakdowns on the Northern Motorway, one in the Victoria Park Tunnel and one on the Harbour Bridge, have been cleared, creating delays.
Constant rain in the City of Sails throughout the day is to blame for many of the delays, with surface flooding forcing motorway lanes to be closed.
As of 5.34pm, MetService reported 58mm of rain had fallen in Auckland today, more than had fallen in the previous 26 days combined at 29mm, according to Niwa.
Those exiting the Sky City carpark during peak travel times were being warned of a 45-minute delay just to get out onto the street.
An NZME employee said she had spent 40 minutes sitting in her car and described the situation as "pandemonium".
Traffic lights are broken at the intersection of Esmonde Rd and Fred Thomas Dr in Takapuna, the NZTA said, asking motorists to take care.
Meanwhile, NZ Transport Agency said surface flooding had been disruptive all day throughout Auckland's motorway network.
Congestion is heavy but easing from Tristram Ave to the Upper Harbour Highway moving northbound along the Northern Motorway. Heading southbound, there was a queue for the Cook St off-ramp.
Traffic is heavy at Manukau, easing through Takanini, southbound on the Southern Motorway and heavy between Mt Wellington and Greenlane northbound.
On the Northwestern Motorway, there is a queue for the Northern Link southbound and heavy traffic between St Lukes Rd and Lincoln Rd heading northbound.
Those on the Southwestern Motorway can expect heavy traffic approaching the Southern Link heading southbound and heavy traffic between Massey Rd and Neilson St northbound.
Both directions of the Upper Harbour Highway are free-flowing, both east and westbound, NZTA reported at 6.35pm.
Surface flooding has been affecting other roads across the upper North Island today.
Slips and a rapid rise of water levels in streams and rivers were also possible in Northland, Auckland, Waikato and Bay of Plenty today, MetService warned.
A vehicle breakdown in the Waterview tunnel was also causing delays earlier, with drivers asked to merge right if possible.
An earlier breakdown that was blocking the left lane of the Northern motorway just after the Victoria Park tunnel heading north has been cleared.
Elsewhere, a severe weather warning is in place for State Highway 1 around the Tongariro National park and State Highway 5 from Napier to Taupo.
NZTA said heavy rain and snow overnight might impact driving conditions and urged motorists to drive to the conditions.
