A truck has ploughed through a primary school in Sydney's south after failing to stop at an intersection and crashing into several cars. 

Emergency crews worked to free the trapped male truck driver after he crashed into a classroom at St Joseph's Catholic Primary School in Moorebank.

The driver reportedly failed to stop at an intersection on Newbridge Road and Nuwarra Road at about 1pm on Saturday. 

Police will probe whether the driver had a medical episode before the crash. 

'He's gone off the road, crashed into the gate at the local church and then finished underneath a classroom at the school,' Sergeant Shaun Colls told 7News.

'Thankfully, it is a weekend (so no children were at the school),' he said. 

Up to a dozen cars were reportedly struck as the truck entered the intersection with four people - including the truck driver - treated by paramedics at the scene. 

A woman in her 20s, a man in his 30s and a woman in her 70s were all taken to Liverpool Hospital. 

Nobody, including the driver, was seriously injured.

The truck was stuck inside a classroom at the school, with traffic brought to a standstill due to the scale of the crashes.

NSW Fire and Rescue were at the scene to assess the building's structural safety.

Police are asking anyone with information or footage of the crash to come forward.