Consumer NZ is questioning a promotion by Woolworths that gives $30 off to only some customers.

The supermarket chain is marking 30 years of online shopping.

As part of that, it is offering a $30 discount to people who have not shopped online before, if they spend more than $100, and people who have previously made small, regular transactions.

It is not offering the code to higher-spending customers.

"Online shopping began as a trial with 100 customers in 1996. Today more than 160,000 online orders are processed every week by Woolworths NZ, or around eight million per year," a spokesperson said.

"We are bringing the celebrations to our customers with online deals, including 30% off selected products, $30 off for customers placing their first online orders, and more."

Consumer NZ spokesperson Jessica Walker said there would be many loyal customers who could not access the $30-off promotion.

"Across many sectors we see the best offers being available for new customers, which means loyal customers end up paying a loyalty tax. This is the case across power, internet, mobile phones and can be true for insurance too. Loyalty tends to reward a business more than the customer," she said.

"It's worth noting that the more people that shop online, the more data that Woolworths can collect about its customers shopping habits. $30 is a cheap price to pay for ongoing shopping data about new customers.

"The widespread collection of personalised shopping data could end up being detrimental to customers, as it greatly increases supermarkets' understanding of how aggressively items can be priced to individuals."