Police found Kuol Akut unconscious on Hainsworth Avenue about 11.45pm.
Officers had attended the area earlier in the night but identified no anti-social issues.
The party finished about 11pm, with about 180 guests leaving the house.
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READ NOWKuol was last seen riding a white BMX bike, which has not been recovered, north along Hainsworth Avenue.
People standing nearby left before police could speak to them.
Officers are still interviewing guests and reviewing CCTV and mobile footage.
A male who emailed Crime Stoppers about 8pm with specific information about Kuol has been asked to contact police.
In a separate incident in the same area, police also located an 18-year-old on the footpath near Meakers Way in Girrawheen, believed to have been a victim of assault.
He was taken to Sir Charles Gairdner Hospital on Friday night and discharged the next day.
If you have any information on either of these incidents, please call Crime Stoppers on 1800 333 000 where calls are anonymous.