A man who was captured on camera carrying out a "forceful and vicious" attack on a mentally ill woman on a Sydney train has been jailed for at least nine years.

Distressing footage has emerged of David Marlin shaking the 34-year-old woman's hand, seconds before knocking her down with a single, explosive punch.

CCTV footage of the attack, obtained by Channel Nine, shows the woman's head being violently flung back by the force of Marlin's punch.

Marlin, who has previously been jailed for rape, then sexually assaulted the woman on the train carriage near Ingleburn in Sydney's south-west last year.