"He said he had a quarrel with my mother the night before. My mother did not believe the amount they were receiving in Comprehensive Social Security Assistance and suspected my father had pocketed some of the money," Wu said.

"He said my mother held a knife in front of him. He shouted at her to stop but she didn't. He fought back and a fight followed."

The son said his father claimed to have used the hammer in self-defence.

Wu said his father was calm when he described what happened. He told him to call the police but Woo replied: "No way - there is no use". The elderly man became agitated and said he wanted to commit suicide. Wu then called the police.

Wu's wife, Mai Zhuolan, who was also present when her father-in-law confessed, said Woo had mentioned that the fight started with "an exchange of throwing banana skins at each other". But events then took a chilling turn.

Prosecutor Bernard Ryan told the court that police found Lai in a pool of blood on her bed.

A pathologist put the killing at between 10pm and midnight on May 26. She had 33 wounds to her head, face and neck, and injuries on her upper limbs. Her skull and jaw were fractured.

Social worker Chan Shuk-yi said Lai and her daughter approached a social welfare centre on May 14 last year. Lai said she did not get on with her husband and wanted to check how much public assistance they received. She also wanted to know how much she would get if she applied for assistance on her own.

Chan said the couple were eligible for about HK$7,300 a month - and received HK$6,500 after rent and fees. Chan visited their flat on the morning of the killing and Woo told her he gave his wife HK$4,000 a month.

The case continues today.