Figures from Statistics New Zealand suggest many retailers did not pass on an increase of 2.5% in GST to shoppers last year.
A survey of half of the items at shops in the final three months of last year show they did not reflect the higher mark up, which took effect on 1 October. The Goods and Services Tax was previously 12.5%.
Living costs rose 2.3% in the final three months of 2010, as some retailers passed on the GST and petrol prices surged.
But the Retailers Association says other retailers were reluctant to follow.
Retailer Briscoes says the decision reflected the tough trading conditions at the end of last year.
Managing director Rod Duke says that with petrol prices at $2 per litre, retailers are in for another tough year.