NSW Police are investigating a fire involving a truck on the Bywong overnight.

At about 9.30pm, ACT and NSW Emergency Services responded to reports of a truck fire on the Federal Highway.

Upon arrival the truck cab was well alight with fire spreading into the cargo area.

Firefighters quickly knocked down the fire containing it to the truck and a small amount of grassland.

The Federal Highway was partially closed while emergency services were in attendance.

It's believed the truck was at the location for an extended period of time and NSW Police have commenced an investigation into the fire.