A Sydney judge has condemned the vow of chastity as a "cruel requirement" as he sentenced a Catholic priest to at least six months in jail for grooming a girl for sex.

The parish priest was arrested last August when he went to the Parramatta Stadium car park to meet a person he believed was a 13-year-old girl he had been chatting with for three weeks on the internet.

Father Robert MacGregor Fuller had used a webcam to broadcast himself masturbating to 'Katie', who was in fact an undercover detective.

Fuller, 54, had served for six years at All Saints church in Liverpool, in south-west Sydney.

He was today handed a maximum jail sentence of 18 months, with a non-parole period of six months, after pleading guilty in Parramatta District Court.

In sentencing submissions today, the priest told the court the internet relationship was a fantasy and a game, and he needed to release energy.

Handing down his sentence, Justice Hughes said Fuller had been sexually frustrated after a life of celibacy. He described the vow of chastity as "agonising" and "cruel".

But the judge said children were vulnerable and sexually innocent and "for an adult to exploit them is repugnant."

Justice Hughes said other people with pressures of life managed to escape those through legal forms of recreation.

Fuller was suspended by the Archdiocese of Sydney when he was arrested.