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The 28-year-old man will spend a fifth night in hospital with several facial fractures and a broken arm after the incident in the suburb of Trigg.
Detectives believe a potato gun may have potentially been used.
The scooter rider and a friend were travelling south on rental scooters along the footpath of West Coast Drive on Friday night.
As they approached Mettams Pool at about 9:30pm, a person in a white sedan allegedly launched the potato and hit the 28-year-old in the face.