A 15-year-old boy has been arrested after a woman was sexually assaulted while walking through a park in Forrestfield.

The boy is assisting detectives from the sex assault squad as they investigate an attack from Thursday afternoon.

Police said a woman was walking through Juniper Reserve when she was attacked by an unknown male between 5.15pm and 5.25pm, before he fled on a bike in the direction of Tallerack Way.

In an update on Friday, police said a 15-year-old male was in custody.

No charges had been laid at the time of publication.

Police are still investigating the incident and urge anyone who witnessed the attack to contact Crime Stoppers on 1800 333 000 or online.

Dash-cam footage, mobile phone or CCTV from nearby homes can be supplied directly to police here.