Arthur Alliband had spent barely a few hours outside prison walls before he allegedly sexually assaulted another teenage girl.

The convicted sex offender was on his way from prison on Monday afternoon, after being granted parole, when he befriended a 16-year-old girl on a train.

It will be alleged Alliband went with the girl to Best Road Reserve in Seven Hills in Sydney's west, buying her alcohol from a bottle shop along the way.

Alliband, 26, is accused of having sexual intercourse with the teen without her consent, grabbing her breast and rubbing his fingers on the front of her underwear.