THE former wife of a man jailed for a sickening attack on a woman on a Sydney train has described him as an 'animal' who is finally being punished for a life of assaults.

David Marlin was sentenced to a minimum of nine years in jail after the sexual assault of a woman, 34, who suffers mental illness.

CCTV footage, obtained by 9News, shows Marlin approaching the woman on a train in Sydney's southwest last year.

He extends his hand.

Without looking, the woman offers her hand towards his.

What follows is truly shocking.

Marlin grabs the woman's hand and then hits her across the face immediately afterwards.

She falls back, completely defenceless.

Judge Paul Conlon said the woman was 'simply helpless to protect herself from this forceful, vicious and determined attack.''

Channel Nine reported Marlin had a long list of violent assaults dating back to 1984.

He spent three years in prison in 2006 for rape.

Mr Marlin's ex-wife Margaret Dickson shared details of her own horrifying experience

"I (once) woke to him on top of me, headbutting me into unconscious - my babysitter who was only 15 ran up the stairs screaming, which caused him to stop," she said.

She said she had nicknamed him "Teflon Dave' because 'nothing stuck' - up until yesterday when justice finally caught up with him.