The concrete fell on west bound lane of the Jean Batten Drive intersection and has been cordoned off by police.
A man who works nearby said he and his workmates had been sitting in their tearoom when they felt a large thud.
''It felt like a small earthquake,'' he said.
The man, who would not be identified, said they didn't realise what had happened until traffic began to built up.
It felt like a small earthquake.
Traffic was backing up and motorists are advised to avoid the scene for now.
A reporter at the scene said a large crane was being used to hoist the slab onto a truck.
Traffic was down to one lane but still flowing, he said.
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