The 11-year-old was last seen at a home in Maritime Circuit at Point Cook on Thursday 20 August about 7:30am.
She was possibly wearing a tie-die jumper and black pants.
Nikiya is perceived to be of Pacific Islander appearance, with a solid build, brown eyes and long brown hair with blonde streaks.
Police and family hold concerns for her welfare due to her young age.
Anyone who sees Nikiya or has information on her whereabouts should call Werribee Police Station (03) 9742 9444.