Melbourne Sexual Offences and Child Abuse Investigation Team detectives are investigating an incident at Federation Square earlier this year, Victoria Police say.
A number of boys, aged between 10-12, were using the toilets in Federation Square when they noticed a man taking their photo about 3pm on Wednesday 31 May.
The man has then left the toilets and met up with another man who was outside waiting for him and the pair walked away.
The boys have later reported the incident to police.
The man is perceived to be Asian in appearance and aged in his mid-20s, with tanned skin.
He was wearing a light grey jumper with dark grey dots, black shorts, a Red Bull cap and grey Nike shoes.
The phone is described as being an Apple iPhone with a plain white cover.
Detectives have released an image of a man they believe can assist with their enquiries.
Anyone with information is urged to contact Crime Stoppers.