A car has burst into flames just off Auckland's southwestern motorway.

Photos posted to Twitter by well-known scientist Michelle Dickinson showed a car engulfed by fire.

"The northbound Queenstown Rd off-ramp. Pass the scene with care and expect delays as emergency services work to CLEAR the incident as quickly as possible. Expect reduced visibility in the area and plan ahead," NZTA said.

Dickinson's husband Joe Davis was on the Southern Motorway where he had earlier noticed overhead signs warning of an incident ahead.

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"I was expecting traffic, but I wasn't expecting to see a car fully involved with fire on the side of the road," he said.

As Davis approached, large amounts of smoke were billowing up near the Queenstown Rd off-ramp.

Then as he came near the car, he saw it ablaze with flames "a couple of metres" high.

"I don't know if it was the owner, but a man was standing 100m down the road on his phone," Davis said.

"I hope everyone got out safely."

Emergency teams were not yet at the scene when Davis passed by the car.

Motorists were attempting to give the flaming wreck room and pass by carefully.

Racing along on the other side of the motorway, a fire truck from "our awesome fire service" was hurrying to get to the scene, Davis said.

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He said there were no signs the flaming car had been involved in a crash with another car.

"Not that I could see; it was literally a vehicle pulled right to the side of the road," he said.