Countdown has been ordered to withdraw advertisements for a "one-day sale'' that ran online for 21 hours over two days.
The television ad included a voiceover stating the sale was on "only tomorrow and only at Countdown''.
However, fine print at the bottom of the screen clarified the sale prices were valid "in-store on Wednesday 18 September 2013 only or online from 4pm Tuesday 17 - 1pm Wednesday 18 September 2013''.
A complaint to the Advertising Standards Authority from T Orwin said people would expect to be able to purchase the goods on the Wednesday referred to in the ad, rather than the 21-hour period specified in the fine print.
The response from Progressive Enterprises said the promotion was valid all day Wednesday in-store, but differed for online because 1pm was the cut off for same-day delivery.
The promotion had since been extended to 4pm on Wednesdays, to allow a full 24-hour period.
"In any event we do not consider that 21 hours of online promotion is misleading given the clarity of the disclaimer and the fact that most of our stores are only open for 14-16 hours per day anyway.''
A majority decision of the complaints board found the ad was likely to mislead or deceive the consumer about the terms and conditions of the sale.
The ad was to be removed.
Countdown is appealing against the decision.