In its written decision the Complaints Board said a minority of the board was of the view the advertisement was clear in positioning the claim as applying to My Tax's customers and was unlikely to mislead consumers.
However, the majority of the board said the advertisement implied the claim applied to all New Zealanders, not just My Tax customers.
It said this was likely to mislead consumers into thinking 90 per cent of New Zealanders were overpaying on their tax.
The board said without clarification the advertisement was likely to mislead consumers, and the complaint was upheld.
My Tax were not immediately available for comment.
- NZME