Lily, aged 10, and Pearl, 8, were last seen at a petrol station in South Auckland at 11pm on Monday, January 2.
The Toyota Camry they were traveling in was seen again in New Plymouth on Tuesday morning, before a woman wanted in connection with their disappearance was found alone in Wellington that night.
She was arrested around 9:45 p.m. and refuses to tell police where the girls are.
Police then spent time searching vehicles parked in central Wellington, but believe Pearl and Lily could be anywhere between Auckland and Wellington.
Detective Senior Sergeant Kevan Verry said the woman met people through Facebook during the time police were looking for her and the girls.
“It’s possible she left Pearl and Lily with one of these people.
“We believe these contacts may have been misled or unaware of the true circumstances of this case, or the serious concerns being expressed for the well-being of the children.”
Police are asking anyone with information to call 911. They also want to hear if a silver 1994 Toyota Camry with license plate CLR45 has been seen.
Detective Sergeant Verry said their focus was on locating Lily and Pearl safe and sound.
Meanwhile, the woman is due to appear in Wellington District Court today on charges of breaching a parental order.