A man has been left in hospital after trying to stop a herd of cows attacking a jogger in New Zealand.

Witnesses say the good Samaritan was trampled and lost chunks of flesh when he jumped in to protect a woman being set upon by a group of about two dozen cows at an Auckland park on Sunday.

Bystander Mike Small told the NZ Herald he heard the woman screaming as she was surrounded and attacked by the bovine.

"All of a sudden this guy appeared and helped her to her feet and then the cows knocked him to the ground and trampled him," he said.

"There's no nice way to put it: He got f****d up," Small said.

The rescuer - who managed to get himself over a nearby fence - was on Monday recovering in a stable condition, a Middlemore Hospital spokeswoman said.

The jogger being attacked, Aimee Wedgwood, told the Herald she used some "wild karate" to try to keep the cows at bay before being charged, but was only left with a few bruises.

"They were following me with their eyes. I was actually on my third loop of the park, I'd passed them two times before," she said.

"I backed off the path then they all came around me from all directions."