The two spoke to each other briefly when the suspect asked the victim to help him look for his lost mobile phone in an adjacent toilet.
The suspect is described as being of Caucasian appearance, about 165cm tall, medium build, aged in his early- to mid-teens with brown hair.
Officers said he was wearing a black T-shirt and shorts.
1800 RESPECT on 1800 737 732
National Sexual Abuse and Redress Support Service on 1800 211 028
Kids Helpline on 1800 55 1800 (for people aged 5 to 25)