Police first received a missing person report on Saturday morning, and confirmed that the North Shore teen had not been seen since Friday evening.
On Sunday night, Mia's mother Tracey Stewart made a desperate plea on social media appealing for any sightings of her daughter, who she had not seen all weekend.
She said the family was "absolutely sick with worry".
"This is by far the hardest thing I have had to write and we do not want or need anyone's judgement, gossip or speculation," she wrote on Facebook.
"We just need your help."
Stewart said Mia had been missing from the North Shore/Hibiscus Coast area. The teen had not used her phone or been online since the very early hours of Saturday morning.
The post went viral on Facebook with more than 44,000 people sharing her message and an outpouring of supportive messages rushing in for the family.
Detective Senior Sergeant Nadene Richmond, of Waitemata North, said today police were continuing to investigate Mia's whereabouts.
A team of police based in Orewa were “working on the matter”.
“Police are working through inquiries at this stage and we are following up reported sightings of Mia.
“If you have seen Mia Cooper, please contact police on (09) 426 4555.”
