A mother and her 10-year-old son have been killed in a road crash near Corrigin, in Western Australia's Wheatbelt, after a semi-trailer ploughed into a parked car.

Key points: The woman and her four children were sitting in the car after it ran out of fuel

The woman and her four children were sitting in the car after it ran out of fuel The grain truck slammed into the parked car, propelling it into a paddock

The grain truck slammed into the parked car, propelling it into a paddock The boy's father only learned of the crash when he was found walking near Corrigin

Three other children from the same family were seriously injured, one critically, and are being treated in hospital.

The woman and her four children were sitting in a Holden Commodore parked on the side of the Brookton-Corrigin Road on Sunday night, about 28 kilometres from Corrigin.

The car, which had been towing a trailer, had run out of petrol and the male driver — the children's father — had left to walk to Corrigin to get fuel.

But before he could make it back, the semi-trailer, driven by a 46-year-old man and towing grain trailers, slammed into the parked car around midnight.

A 26-year-old woman and a 10-year-old boy died in the crash. ( ABC News: James Carmody )

The car and trailer were shunted into a paddock by the force of the impact.

The woman and the boy died at the scene, while the three other children — boys aged 8 and 5 and a 3-year-old girl — were flown to Perth Children's Hospital by helicopter.

A spokesman for the hospital said the five-year-old was in a critical condition, while the others were serious but stable.

The father, a 27-year-old man, only learned of the crash when police found him walking along the Brookton-Corrigin Road, about 17 kilometres away.

Police examining 'tragic scene'

The family are believed to be locals from the Corrigin area, about 220 kilometres east of Perth.

Police have closed roads in the area and remain at the scene.

Roads in the area are closed following the fatal crash. ( ABC News: James Carmody )

Detective Senior Sergeant Steve Potter from Major Crash said investigators would look at whether the condition of the road was a factor.

"It's a very tragic scene," he told ABC Radio Perth.

Great Southern district superintendent Ian Clarke said police were interviewing witnesses to the crash as well as the truck driver, who did not sustain any physical injuries.

"As you can imagine, the scene was quite catastrophic, with a fully laden truck crashing into a car," he said.

"It has devastating consequences for everyone involved — family, obviously the people directly involved and emergency services — so we really hate seeing anything like this, it's a terrible situation all round.

"It's a terribly tragic thing for the Corrigin community."

The Brookton-Corrigin Road remains closed in both directions between Rigby Road and Bulyee Road.

Corrigin shire president Lyn Baker said support was being provided to children at the local school.

"I believe there's an emergency meeting happening at the moment so that they can work out how best to support the other children at the school, and of course the teachers that were involved with the child," she said.

"It's a time when the whole community needs to just get together and support [everyone], certainly the family."