A Sydney corporate lawyer who crashed into and then attacked a learner motorcyclist will argue that he was mentally ill at the time and should not be sentenced under NSW criminal laws.

Murray Bede McArdle, who was until recently the chief legal officer at engineering firm UGL, pleaded guilty to common assault and malicious damage over the incident on the Pacific Highway at North Sydney in July last year.

The 55-year-old hit communications consultant Alex Chapman's motorbike from behind and then assaulted him by grabbing him by the throat.

Mr Chapman had been on his learner's licence for only five days.