Man hit trying to cross Sydney motorway

A man who tried to cross Sydney's busy M7 motorway has been hit by a truck and seriously injured.

A man is in a critical condition in hospital after being struck by a truck as he tried to cross the busy M7 motorway in Sydney's northwest.

Paramedics and a Careflight doctor treated the man, believed to be aged in his 50s, for head, chest, arm and leg injuries at Bella Vista late on Sunday morning.

He was placed in an induced coma and taken to Westmead Hospital in a critical condition, Careflight said.