A car that caught fire on State Highway 1 today may have suffered a mechanical fault, police say.

Heavy traffic has eased following the incident north of the Harbour Bridge today, New Zealand Transport Association Auckland says.

Motorist Richard Morse was driving past the car when it "almost exploded" in flames.

He could see a police car and a man standing further along the motorway but it was hard to make things out because of the smoke, Mr Morse said.

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"We couldn't see it for a while...there was a little bit of smoke and I thought 'That could be a car fire' and as we got closer it got more, and then we came round the corner it was fully alight and a kind of fireball came up.

"There was so much smoke billowing off it we couldn't see over there that well," he said.

The driver was in the vehicle when it caught fire and was able to pull over to the kerb, Waitakere Police Senior Sergeant Ross Hunter said.

It has been towed from the scene, he said.

No one was hurt in the incident and it is not being treated as suspicious.

"The motorway guys will be looking into it but if it was just a straight car fire then, unless it's suspicious which is highly unlikely if the guy was driving it, they wouldn't go much further than that. It's just one of those things," he said.

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Photo / Hanna Morse
Photo / Hanna Morse
Photo / Antony Gray
Photo / Antony Gray
Photo / Antony Gray
Photo / Antony Gray
Photo / Antony Gray
Photo / Antony Gray