He has leased the paddocks between Heads Rd and Puriri St from Whanganui District Council since 2009 and uses them once a year as a finishing block for the cattle he raises on his 16-hectare Okoia farm.
The 26 cattle he had grazing the paddocks were to be his main source of income for the year when they were sent to the works in the coming weeks.
Mr Kerwin had been working at the block on Monday afternoon; the vet called to assess the injured animals believed the four dead cattle would have been killed on Monday night.
It was people working in the community garden that borders the land who noticed the butchered and injured animals and contacted the council yesterday morning.
Mr Kerwin said the paddocks behind Heads Rd were protected by security cameras at adjacent businesses, but the paddock behind Puriri St that his cattle were in was vulnerable to poachers.
He said he would shift the remaining 19 animals back to his Okoia farm and was offering a $500 reward for the conviction of those involved. Those with information should call 027-496 4327.
The police had been informed of the incident, although Mr Kerwin was still waiting to hear back from them yesterday afternoon.