Wollongong Local Court magistrate Les Mabbutt said the doctor's credibility during court proceedings was "in tatters" as he found him guilty of indecent assault.

He said Bosanquet's actions in the lead-up to the assault had been a "deliberate deception of (the victim) and his father".

Bosanquet had pleaded not guilty to fondling the victim, then aged 19, in his northern Illawarra home on December 19, 2000.

He had previously admitted inciting the victim and two other patients to perform an indecent act between 2000 and 2007.

The oral and maxillofacial surgeon, who has been struck off the medical register, devised a bogus medical study to convince the victims to masturbate in front of him.

During his evidence yesterday, Bosanquet, 55, of Jamberoo, said he could not explain his motivation for inciting the teenagers except to say that he had been "sick".

At the time of the offences he did not think his actions were wrong because he had verbal agreement from the victims to participate in the bogus study, however, his understanding had changed since receiving 12 months of psychological and psychiatric therapy.

In his verdict, Mr Mabbutt rejected Bosanquet's evidence that he was not sexually attracted to the 19-year-old and that he had not been motivated by sexual gratification.

Mr Mabbutt said Bosanquet had used his standing as a respected surgeon over his victim, who had been referred to him for facial surgery following an assault and also had a procedure done in relation to his wisdom teeth.

Bosanquet convinced the victim and his father that he was conducting a medical trial to measure the blood pressure of men before, during and after masturbation.

He groomed the victim with promises of cash payments and by taking him on a joy ride in his luxury car.

The teenager agreed to participate in the "trial" and when Bosanquet visited his home the pair went to the victim's bedroom, where Bosanquet took a blood sample and then asked the teenager to masturbate.

When he could not get an erection, Bosanquet told him to close his eyes and think of something that would arouse him.

The surgeon then indecently assaulted the teenager.

The court heard the victim did not report the incident until years later, because he thought it was so bizarre no-one would believe him.

Bosanquet will be sentenced on all charges next year.