The presence of a technician's semen on his daughter's shorts was "very persuasive" in proving his guilt, a High Court judge said yesterday, as he convicted the man of raping the girl when she was 14 years old.

The 35-year-old man, who cannot be named to protect his daughter's identity, had claimed trial to two charges of raping his biological daughter in the family flat in Punggol.

During the trial, the girl testified that her father had raped her eight times over two months.

On Nov 19, 2014, he forced his way into the locked common toilet while she was changing and raped her, she said.

He raped her for the last time on the night of Jan 9, 2015, she said. The assault took place in her bedroom, she added, while her mother was out playing mahjong and her two younger siblings were playing games in the master bedroom.

The girl, meanwhile, had told her then boyfriend and schoolmate about her father's actions, but was reluctant to go to the police.

The boy arranged for her to see the school counsellor, and a police report was eventually made on Jan 13, 2015. That night, the police went to the flat where they seized a pair of shorts the girl had worn.

A forensic analysis found her father's semen on the crotch area.

The court heard that the girl's family did not believe her and blamed her for getting her father into trouble. Now 18, she lives with another family.

In court, her father - as well as her mother, who testified for him - depicted her as a "rebellious" girl who had fabricated the allegations.

However, Deputy Public Prosecutors Winston Man and Chee Ee Ling argued that the girl had no reason to falsely implicate her father and disrupt her life so drastically. This was underscored by her reluctance to report the matter.

Ultimately, the prosecutors said, the forensic evidence was "damning" and the accused could not explain how his semen had ended up on her shorts.

Justice Lee Seiu Kin, in his verdict yesterday, noted that this has been a "difficult" case in that the charges are very disturbing as they involve a daughter accusing her biological father of rape.

Still, the judge found that the prosecution has proved its case beyond reasonable doubt.

Justice Lee noted that the semen was found in an area of the girl's shorts that was consistent with her version of events.

The judge was also persuaded by the girl's unusually compelling testimony. Her "version of events was given with a level of detail that can only come from a person telling the truth", he said. He added that her friends and school counsellor corroborated her account of how the offences came to light.

The case was adjourned for sentencing arguments and bail for the accused was increased from $40,000 to $60,000. Each charge of rape carries up to 20 years' jail, and caning or a fine.