An eight-year-old boy has been preyed on by a man who took photos of him from underneath a toilet cubicle at one of Melbourne's busiest shopping centres.

The young boy and his father were in the toilets at the Westfield Southland shopping centre about 4pm on Saturday, December 28, when a man entered the cubicle next to the boy and started taking photos of him with his mobile phone from underneath the dividing wall.

He then left the toilets.

The man is described as fair-skinned, aged between his late 20s to early 30s, with a stocky build. He was wearing a dark cap, dark-coloured T-shirt and blue jeans.

He was also sporting a distinctive, seaweed-like tattoo on his upper arm.

The mobile phone had a black case with a picture of a white skull and fire on it.

Police have released an image of a man they would like to speak to.

Anyone with information is asked to phone Crime Stoppers on 1800 333 000.