Truck driver in fatal M5 crash refused bail

The truck driver accused of killing a 10-year-old boy during a deadly pile-up on Sydney's M5 will spend Christmas behinds bars after being refused bail.

Pushminder Grewal applied for bail today at Bankstown Court, but the judge refused, calling the prosecution's case "extremely strong".

The 26-year-old, who is a father, is facing five charges including dangerous driving causing death after the horrific crash on Wednesday that killed a young boy on his way to school.

The boy's 13-year-old sister, mother, and another woman were also injured in the accident.

The truck driver was transporting a full load of wooden pallets near Riverwood when his truck allegedly slammed into slow-moving traffic during the morning peak.

Mr Grewal's lawyer claimed the truck's brakes failed, however the prosecution told the court the crash investigation unit tested the brakes and "they were fully functional at the time".

"It's not said to be speeding, or use of the phone, or use of drugs, nothing of that nature," Elias Tabchouri said outside court.

Police have seized the driver's mobile phone and reviewed the truck's dash-cam footage, which allegedly shows the truck swerving from one lane to another and crossing a median strip.

The court heard brakes were allegedly not applied until the truck smashed into the back of the Subaru carrying the boy and his family.

Mr Grewal's previous driving offences include driving an unlicensed and unregistered vehicle and refusing to do a breath test.

The Indian national is now facing up to 14 years behind bars. He will front court again on January 22.