A young boy has survived a motorbike accident that killed his father north-west of Sydney, police say.

The pair were riding along Putty Road in Mellong when they crashed on Sunday morning.

The father, aged in his mid-30s, died at the scene while his son, 12, was taken to The Children's Hospital at Westmead with serious hip and leg injuries.

On Sunday night he was in a serious but stable condition.

The pair were at the front of a group of motorcyclists out for a morning ride when they crashed.

Putty Road was closed for several hours as an investigation into the cause got underway.

There have been several fatal crashes on Putty Road at Mellong in recent years - including in November 2011 when two motorcyclists died after hitting a ute.

AAP