A road train carrying bales of hay from Western Australia to drought-stricken farmers in New South Wales has caught fire on the Nullarbor, with searing temperatures in the area nearing 50 degrees Celsius.

Key points: The driver and his partner escaped unharmed

The driver and his partner escaped unharmed It is not yet known what caused the fire

It is not yet known what caused the fire The rest of the convoy is due to arrive in the Cobar region of NSW on Australia Day

The vehicle — one of nearly 50 trucks packed with 3,000 bales of donated hay and fodder — caught fire just west of Madura roadhouse on the Eyre Highway on Wednesday.

The driver and his partner escaped uninjured.

The area has been enduring extreme heat — up to 49C — and there is currently a fire alert in effect.

The road train caught fire near Madura roadhouse. ( Supplied: Sarsha Jade Lander )

Convoy organiser Sam Starcevich said it was not yet known why the vehicle caught fire.

"The driver pulled up to have a break, stretch his legs and as he went to jump out he noticed that his trailer or the hay behind the cab had caught on fire," Ms Starcevich said.

"We're just so thankful that they are both OK.

"He was devastated that he was not going to get the hay there … and we're going, 'No, as long as you are OK that is the only thing that matters'."

Ms Starcevich said the roadhouse owners had stepped in to help the couple, providing accommodation for them free of charge.

"It's just that good spirit of everyone helping each other out in times of need," she said.

The donated hay and fodder will help up to 400 farmers in New South Wales. ( ABC News: Tara de Landgrafft )

The convoy left Norseman on Tuesday, headed to the Cobar region of central-west NSW.

It is due to arrive on Australia Day.