By Stephen Wright

WELLINGTON, New Zealand--About a third of mobile phone users are paying more than they need to, New Zealand's competition regulator said Thursday, calling on telecommunications companies to increase billing transparency.

A study of nearly 80,000 mobile phone bills found 7.0% of people could save an average of about 580 New Zealand dollars ($390) a year by moving to a cheaper plan that matches their usage, the Commerce Commission said. Another quarter could save about NZ$139 a year, it said.

"Our work suggests that some consumers are significantly overspending on their mobile plans due to transparency and inertia problems in the market," the regulator said.

In an open letter to three mobile companies--Spark New Zealand Ltd., 2degrees and Infratil Ltd.-owned Vodafone New Zealand--the regulator said it expects them to address overspending.

The companies can increase usage information for customers and implement measures that help keep people on plans that reflect their actual requirements, it said.

Work will also begin, the regulator said, on developing a "consumer data right" that will help people share their mobile usage, spending and product information with competitors and comparison services.

The Commerce Commission research also found that 54% of people had not changed their mobile phone company in the last five years while more than two thirds never or rarely considered their mobile plan options. 



  Write to Stephen Wright at stephen.wright@wsj.com