The man accused of murdering his former mistress has broken down while describing how a struggle in her Sydney apartment turned violent.

Accused murderer Paul Mulvihill has broken down recalling how his former mistress was left with blood pouring from her neck after the two struggled violently over a “massive” knife.

In emotional testimony in the NSW Supreme Court, Mulvihill said he was acting in self-defence when Rachelle Yeo was stabbed with a large kitchen knife at her apartment in Sydney’s north in July 2012.

The father of three said things turned violent after she slapped him for calling her “the most self-centred person I have ever met”, who could never handle the responsibility of being a parent.

Ms Yeo then reached for a “very large stainless-steel knife” and started slashing and lunging at him with it, he said.

As the two wrestled for control of the knife, Mulvihill said he thought he could see “in her eyes” he was in “massive trouble”.

“She was so angry … it was coming for me,” he said.

“We were literally bouncing off the walls.”

Mulvihill, 46, said Ms Yeo then lost her balance as the stoush continued, yanking him by his jacket to the ground as she fell.

When she moved, he noticed she was bleeding from one side of her body.

Neighbours started banging on the front door as the fight continued, with Ms Yeo urging them to call the police, Mulvihill said.

He said Ms Yeo, 31, was still “frantic” even after he effectively pinned her down, so in a bid to “defuse” the situation, he punched her in the face.

He choked up describing how the blow stunned her and she dropped the knife.

But when she moved to seize it again, Mulvihill said he tackled her and braced himself against her when her arm suddenly gave way and the blade plunged into her neck.

“All the blood started coming out … I looked at her and I knew it was bad,” he said, breaking down.

He then re-enacted the struggle before the jury, with a member of his legal team standing in for Ms Yeo, with a ruler as the knife.

He said he never meant to hurt her when he went to her unit to get “closure” following their break-up.

After leaving in a panic while Ms Yeo had blood pouring from her neck, Mulvihill said he called his wife and said words to the effect of “She has gone out of our lives”.

When pressed by crown prosecutor Maria Cinque what he meant, he said he was trying to say there was “no way in the world” he’d have anything to do with Ms Yeo again.

“Mr Mulvihill, that’s the reason you went there,” she posed.

“That’s not the reason, Ms Crown,” he replied.

Mulvihill has pleaded not guilty to murdering his former girlfriend.

The trial continues.