A 37-year-old man has been charged after allegedly assaulting UNSW security guards in two separate incidents this week.

On Tuesday afternoon security guard questioned a man who was walking around the Reid campus before being asked to leave.

Later that evening security officers again found the man on the university grounds, where police allege one guard was spat on, and the man threatened to kill them before leaving the campus.

On Wednesday afternoon the man returned yet again to the university where he allegedly spat on another security guard.

Police later arrested the man. A search of his of his belongings revealed a 10-centimetre knife.

He is charged with four counts of common assault, two counts of trespass on school premises and two counts of possessing a knife without reasonable excuse.