When the disgruntled cow began charging towards a school, police said they had no choice but to shoot the animal.
An officer fired one shot at the cow at a close range but this proved ineffective.
Several more gunshots were fired at close range, and the cow was eventually killed at Alma Road, near the intersection with Moore Avenue.
Senior Sergeant Paul Baskett said the incident happened at a busy time in full view of the public.
"We appreciate this must have been a traumatic event to have witnessed.
"However public safety will always be our first consideration in these situations and our staff acted based on their immediate safety concerns at the time.
"This was a big animal and had they not acted there was a real risk that it could have caused serious injury."
Whanganui resident, Christopher Piper, was just about to sit down to breakfast this morning when he heard the four shots being fired right outside his house.
Fearing the worst, Mr Piper cautiously peered outside to see police and animal control firing at an out-of-control cow on his front lawn.
Mr Piper said the first he knew of the incident was hearing shots fired.
"I was quite surprised to see a cow being shot out here. It was a relief to see it was the police firing the shots and not some gun-wielding idiot."
- With additional reporting from Wanganui Chronicle
