A busy main road in South Auckland has been reopened after being partly closed earlier due to a huge fire at a car yard.

Emergency services were called to a car yard on Great South Road, in Drury, just before 7am.

Large flames and billows of dark smoke could be seen in the sky.

Authorities closed a section of the road - between Parkhaven Drive and Sutton Rd - just before 7.30am.

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It was reopened after 9am.

A spokeswoman for Fire and Emergency NZ said a building was "well-involved'' when firefighters arrived just before 7am.

Four fire trucks were called to the scene.

Witness Jess Wilson, who was travelling from Tuakau to Takanini early this morning, said the building was well up in flames by 7am.

"I saw a lot of black smoke - there was smoke pouring out of the windows.''

She said the police were already at the scene and were doing a great job at directing traffic.

Those heading south were being told to do a U-turn.

Wilson said she saw two men standing outside the building who were visibly upset.

"They were pretty distressed and upset watching the fire.''