Emergency services were called to a semi-trailer alight on the highway, approaching the Snowy Mountains Highway exit at Mount Adrah, about 3.45pm.

Both northbound lanes were closed on the Hume near Parkley Trail and diversions were in place as firefighters tackled the blaze.

At 6.30pm, the NSW Transport Management Centre said one northbound lane of the highway had reopened and diversions have been lifted.

Motorists are advised to continue to allow extra travel time in the area.

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It is understood the truck's trailer, which was carrying a number of cars, is alight and the flames have spread to nearby grass.

No one is reported to have been injured in the fire and it is believed the driver managed to separate the prime mover from the trailer.

Police, firefighters and paramedics are on the scene.

Meanwhile, motorists using the Snowy Mountains Highway have been warned of a reduced speed limit due to road surface damage at several locations between the Hume Highway and Yarrangobilly Caves Road.

A speed limit of 80km/h is expected to remain in place at affected locations until the road surface is repaired, the NSW Transport Management Centre said.

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