An apparent conspiracy theorist has stabbed multiple people after a two-car crash on a major Sydney highway

Two cars collided about 9 this morning in south Sydney before the driver of one car allegedly began slashing at his passenger and good Samaritans who rushed to try to help the distressed woman.

Cronulla Sharks chairman Steve Mace says he came face-to-face with the knife-wielding man after the crash happened right in front of him.

He ran to the passenger's aid after hearing her screams but struggled to rip open the door to save her from the attack.

A man's now in custody.