A motorist has filmed the dramatic moment a truck grazed the inside wall of Sydney’s Airport Tunnel, setting off the sprinklers.

The truck, which appears to be over the height limit, skims the edge and the top of the tunnel creating a wall of sparks and sending smoke flying from the truck.

"Numerous" sprinkler heads inside the tunnel were ripped off when the impact occurred around 6.45am, a NSW Fire & Rescue spokesman said.

The tunnel’s sprinklers were then set off, drenching the road with water. They have since been turned off.

Traffic delays in Mascot. (9NEWS)

Traffic delays were heavy in Mascot, but have since returned to "normal". (9NEWS)

Lanes were temporarily closed at General Holmes Drive due to water on the road, but Transport Management Centre spokeswoman Kristen Forbes said traffic was back to normal.

“There might be repairs to the tunnel’s sprinkler system after the afternoon peak, but at the moment things are running as normal,” she told 9news.com.au.

Cyclist John Sunde posted an image of the aftermath of the sprinkler set-off on Instagram, commenting he’d had an “early shower”.

https://www.instagram.com/p/BTFb9RBAJM_/

© Nine Digital Pty Ltd 2017

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