PhD candidate Jeremy Karl Day spent less than five weeks living at the Mount Ousley property before a bizarre stoush with one housemate caused him to make quick plans to move out.

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The housemate testified against Day in in Wollongong Local Court on Monday, claiming the 30-year-old came into his room one morning in January, repeatedly asked about his dog and punched him in the face once ordered out of the room.

But Day gave a very different account, describing the moment he realised he was living in the sharehouse from hell.

He told the court he and other housemates had come to realise the importance of the man's dog to his mental health, and that they took to feeding and caring for it when the man would not.

A marine scientist, he told the court he was about to leave for a two-week research trip when he checked on the wimpering dog out of "moral obligation".

He said the housemate's "putrid, putrid" granny flat had faeces on the walls and floor and the dog had previously been kept unattended in there for long periods, unbeknown to others living there.

"Not knowing he was present in the room, I thought it was going to be another case where the dog was kept in the room for a week with nothing to eat," Day said.

"There was one person who had lived [in the house] for a month and did not know there was a dog on the property. It was madness."

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He said he was surprised to find the man, a hobbyist blacksmith with an interest in making knives and other metalwork, inside the room when he entered. He said the man rushed at him with one hand behind his back, and he feared he was holding a knife.

"He grunted out the one sentence: 'I will kill you'."

"At this point I realised I've moved into a halfway house. I didn't know this person, I didn't know anything about his mental state and I feared for my life."

Day admitted he punched the man three times to the face while backing out of the room.

Magistrate Claire Girotto considered pictures showing the housemate's injuries, which included a facial wound needing seven or eight stitches to face.

But, with the man earlier admitting he suffered a skin infection, Day denied responsibilty for some of his injuries.

"[The room] was absolutely putrid. It was below the standard for a human being to live in, and I believe this is why he had scabs on his face."

Day was found not guilty.

He told the court that he wished, with hindsight, that he had reported the dog's living conditions.

"I regret that. I should have gone to the RSPCA straight away, but I thought I was going to ruin someone's life."

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