They last made contact with family on February 9 and were spotted in Whangarei on February 11.
11-year-old Joshua Lee and sister, Sarah, 13, are missing with their mum, Megan.
A woman, who works at Countdown Whangarei, told the Herald she served the trio in the supermarket about two weeks ago.
"It was Countdown Whangarei about approximately two or three weeks ago. I remember them. I'm a checkout operator and they came to my counter ... I remember the girl with the glasses.
When asked of their disposition, how they appeared, the woman, who did not want to be named, said "they all just seemed normal".
The trio were last known to be travelling in a silver Toyota Corolla station wagon, registration LCG323. Anyone who sees the family is urged to call 111 immediately.