Chilling footage has emerged of the moment a man viciously attacked an unsuspecting woman on a Sydney train.

David Marlin was sentenced to almost a decade behind bars in court today for the sexual assault of the 34-year-old woman last year, Nine News reports.

CCTV shows how Marlin initially approaches the woman on the train near Ingleburn, Sydney's southwest, to shake her hand and then swings his arm to hit her face.

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Chilling footage has emerged of the moment a man viciously attacked an unsuspecting woman on a Sydney train

David Marlin (right) was sentenced to almost a decade behind bars at court today for the sexual assault of the 34-year-old woman last year

CCTV shows how Marlin initially approaching the woman on the train near Ingleburn, Sydney's southwest, to shake her hand and then swings his arm to hit her face

Marlin was sentenced to a minimum of nine years behind bars. It is believed the woman has a mental illness

Marlin was sentenced to a minimum of nine years behind bars.

Sutherland Court was told of Marlin's extensive criminal history, dating back to 1984 to 2002 which include multiple assaults.

Marlin was also jailed for three years in 2006 for rape charges, Nine News reports.

The court also heard the woman who was attacked by Marlin also suffers from a mental illness.

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

His ex-wife, Margaret Dickson, was also at court today and told reporters she was 'happy as all hell' with the outcome.

'He's just an animal,' she said.

Marlin will be eligible for parole in 2023.

Sutherland Court was told of Marlin's extensive criminal history, dating back to 1984 to 2002 which include multiple assaults