Mr Cranstone said the cow even "had a go" at a man in a wheelchair, who was crossing the road on a pedestrian crossing.
Fearing the cow was heading towards Carlton Ave during peak traffic, police shot it on Alma Rd. It took at least nine shots from Mr Cranstone before the cow was killed. No one was injured during the incident, which was over just after 8.30am.
Grant Ogilvie from Central Districts police said if Mr Cranstone had not shot the animal "there was a real risk that the cow could have caused serious injury".
"Staff had to weigh that risk against all other considerations. They had already tried several other options to contain or redirect the animal, which were unsuccessful.
"This incident happened at a busy time in full view of the public, and we understand this must have been a traumatic event to witness."
Mr Ogilvie said the cow was shot with an M4 Bushmaster rifle.
"There was insufficient time to call a vet, given the immediate risk posed, and even if one had been called it is contestable whether this would have changed the outcome of the incident."
Mr Ogilvie declined to say where the animal had come from but said the owner had been found.
"Based on the information available to us at this time, it is unlikely that charges will be considered."
Wanganui vet David Rankin said he could have assisted police in dealing with the cow.
Mr Rankin said he condoned shooting the cow if it was distressed and a public danger.
"But the welfare of the animal should have been taken into consideration," he said.
"If I had been called, I could have assessed the cow and decided what the best option was."
Mr Rankin said he was used to dealing with large animals, and was also a registered gun holder and an experienced hunter.
If the animal had been distressed it would have been all but impossible to use a tranquiliser dart on it, he added.
"We do dart large animals, but when they are calm and contained. When a large animal like a cow is distressed, a sedative doesn't work because the animal is so worked up.
"It would have required a dose so large that it would have been a threat to its life."