Peter Rowlands, 51, a consultant radiologist, had a blazing row with his lover, Fiona Eldean, after she confronted him with evidence that he was having an affair.
She threw his mobile phone into the toilet and tried to flush it away.
Rowlands, who then weighed 21 stones, grabbed his 5ft 3in, seven stone girlfriend and forced her head into the toilet bowl before squirting water on her in revenge.
Eldean, 44, said: 'He is a bully and a liar and I wanted him to be accountable for his actions that day. I don't want him to do this to any other women and I want them to know exactly what he is like.'
Halton and Widnes Magistrates' Courts heard that on New Year's Eve last year Rowlands was about to take a shower at Miss Eldean's house in Widnes while she was downstairs making breakfast.
A text message arrived on his mobile phone from a woman with whom he had been having an affair, and Miss Eldean read it.
She stormed upstairs and told Rowlands to leave her house as their relationship was over.
She threw his phone into the toilet, and in retaliation Rowlands flushed her head.
In the violent struggle that ensued a radiator was knocked off the wall and Miss Eldean ran downstairs to get the house phone.
Rowlands admitted snatching the phone, taking the battery cover off and then sliding it across the floor so she was unable to use it.
Miss Eldean, a florist, then reached for her mobile phone.
Rowlands admitted taking hold of her wrists and 'lowering' her to the ground. It was alleged that Rowlands banged her head on the kitchen floor three times and she retaliated by biting his hand.
Rowlands, of Toxteth, Liverpool admitted cheating on Miss Eldean, whom he met on the internet in June 2007, but denied assault, claiming he had merely restrained her by the wrists to pacify her.
But after a two-day trial he was convicted of two counts of assault. Rowlands, a doctor for 25 years, slumped forward in his seat and put his head in his hands.
Delivering her verdict District Judge Bridget Knight said: 'I hope that your professional body sees this for what it is - confined to one fateful day when a private relationship ended - and that the assault followed from a criminal act and severe provocation.'
Rowlands was fined £1,500 and ordered to pay £790 court costs and £500 compensation.
Dr Peter Williams, medical director of the Royal Liverpool University Hospital, said: 'Dr Rowlands has been employed as a consultant radiologist at the Royal Liverpool and Broadgreen University Hospitals NHS Trust since 1990.
'He is a highly-regarded member of staff whose clinical work is outstanding. The trust will now consider the implications of the case.'
It is understood that Rowlands intends to appeal, Daily Mail reported.