Spark and Vodafone generated the most trade-related complaints to the Commerce Commission last year.
The watchdog's latest report on issues concerning consumers reveals the three most complained-about sectors are telecommunication service providers, motor vehicle traders and domestic appliance retailers.
Almost 90 percent of the nearly 5000 complaints received by the Commerce Commission in 2014 related to the Fair Trading Act, and about a third of those arose from online purchases; that was twice as many as it got regarding purchases at physical stores.
A quarter of trade complaints related to just 24 businesses, including the country's two biggest telecommunication providers, Spark and Vodafone, which generated 128 and 112 complaints respectively.
Also in the top 24 most complained about businesses were ANZ Bank, Air New Zealand, Noel Leeming, sellers on TradeMe and Foodstuffs.
The commission also got complaints relating to the Commerce Act and Credit Contracts and Consumer Finance Act.
Commerce Commission chair Mark Berry said it would use its latest report to target areas where it could best protect consumers.
"We do have a key focus of giving priority to protecting vulnerable consumers and so we've gone through and worked out the highest risk areas for consumer harm," he said.
The complaints did not necessarily translate into breaches of the act but the data was useful for working out which areas to prioritise.
The number of complaints made was up 27 percent on last year.
Spark general manager of customer services Bridgette Dalzell said with two million mobile phone users, and nearly 700,000 broadband customers, the 128 complaints needed to be seen in context.
"Our approach to this is we deal with every one of these individually and look carefully at each one, and we identify where we need to make the changes predominantly around process or upskilling staff and then look at those details of how we're going to improve them," she said.