The Ministry of Primary Industries (MPI) is investigating the death of a young bull dumped next to its owner's letterbox with obscenities spraypainted on its sides.

The incident, which occurred in rural Whangārei on September 8, alarmed animal welfare group HUHA and caught the attention of MPI, which is responsible for investigating animal welfare breaches.

Some of the details of what happened are disputed, but what is uncontested is a bull got through a fence into a neighbouring farm, was caught by the farmer and strapped on to a pallet, then taken a significant distance by tractor and left at the owner's driveway.

It was not immediately clear how, or when, the roughly 300kg animal died.

The bull had a four-letter-word, generally considered the most offensive in the English language, written on its sides in fluorescent pink spray paint.

Brendon Mikkelsen, MPI's northern region animal welfare manager, said staff had visited the site and were working to determine the cause of the animal's death.

MPI's role was to identify possible breaches of the Animal Welfare Act 1999 and hold those responsible to account.

The agency was working closely with police, Mikkelsen said.

HUHA founder Carolyn Press-McKenzie said the bull's "devastated" owner contacted her organisation for advice and notified MPI.

"They were really concerned that maybe it hadn't died in a pleasant way. They wanted to understand how to get help," she said.

Press-McKenzie said she was grateful to MPI for taking the incident seriously.

The bull had no bullet wound and its throat had not been cut, so there was nothing to show the animal had died in a quick, humane manner.

Based on what the bull's owner had told her, Press-McKenzie said the incident was unexpected, with no ongoing dispute, no recurring situations with the bull getting into the neighbour's property, and new fencing.

There were conflicting accounts of where the bull was at the time, including on a riverbank, in a paddock or in with the neighbour's heifers.

Regardless of where the bull was, Press-McKenzie said there were still concerns about its manner of death.

"And when you look at the profanities that have been spraypainted on the animal and the anger of actually driving it all the way two kilometres up the road and dumping it at someone's letterbox, the person who's done this has clearly got themselves in a state, they're angry to a point where I feel they're potentially out of control."

In her view, the best outcome in this case would be some sort of help for the person responsible.

The bull's owner told RNZ her main concern was the bull may have died in an inhumane manner.

She said she would have gone next door immediately and taken the animal home, if the neighbour had called to say it was on his property.

She said the bull should not have been on the neighbour's farm but the situation could have been resolved quickly and humanely with one phone call.

The yearling bull had been hand-reared and was semi-tame, and the fence was brand-new, she said.

When asked when the outcome of the post-mortem examination was expected, MPI said its investigation was ongoing and it had nothing further to add at this point.