A large water rat sneaked into a motor home and attacked a woman as she lay in her bed, after she had mistaken the animal for her three-legged cat.

Marlborough woman Lynne Henderson needed hospital treatment after being attacked in her motor home at Spring Creek Holiday Park on Tuesday.

Mrs Henderson, a permanent resident at the park, told the Marlborough Express she remembered feeling an animal jump on to her bed. She assumed it was her three-legged cat and reached out to pat the animal, which reacted by latching on to her upper lip.

It fled after Mrs Henderson's husband, Neil, turned on the lights.

"It was about nine inches long, very round and cuddly ... well, violent, actually," he told the newspaper.

Mr Henderson fetched an air rifle but he could not get close to the animal because it was "bouncing off the walls and ripped the toilet holder clean off".

He fired one shot into the body of the rat, which "made a hell of a squeal" and ran out the door. They have not seen it since.

Mrs Henderson suffered a pierced upper lip, which bled quite badly at the time. As a precaution, she went to the local hospital for a tetanus injection and a course of antibiotics.

"They [rats] can come into homes looking for food or shelter, or to escape a predator," said Kurt Loklindt, director of Target Pest Control. However, it was unusual for them to attack people.