A man who complained his gas retailer overcharged him for two years was credited more than $2000.

The unidentified gas company was ordered to pay the man, only referred to as Mr Z, $2,555.45 following a Utilities Disputes investigation.

Mr Z alleged he had received conflicting information from his gas retailer which saw him being put on a higher rate than what he should have been on.

A commissioner found the gas retailer gave Mr Z poor customer service when it gave him the wrong information.

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Complaints about billing led issues received by the Utilities Disputes.

The free and independent complaints service released its 2017-18 Annual Report highlighting complaints around electric, gas, broadband and water providers.

Billing complaints made up 60.4 per cent of the 5,232 cases received by Utilities Disputes regarding electricity or gas issues – an increase from 50.7 per cent during 2016-17.

Nanette Moreau, Utilities Disputes commissioner, said sudden high bills, back bills and incorrect bills were among the common issues.

"Sudden high bills, back bills and incorrect bills are among the common issues that can cause a lot of stress, together with disconnections, surges and outages, and switching companies," Moreau said.

"While we can't make decisions about the price of electricity or gas, we can investigate whether bills are accurate, and whether people are on the most appropriate plan. We also look at customer service, and consider whether the communication and information provided was reasonable, for example, whether enough notice was given."

Moreau said while the overall number of complaints remained steady, a smaller number were 'accepted for consideration' by the commissioner.

Just 147 cases of a total 5,528 were accepted for consideration, down from 255 in 2016-17.

"This is because our experienced conciliators have been working hard to resolve complaints up front, facilitating three-way teleconferences with the complainant, the company and our office. The more efficient and timely process is good for both consumers and companies," Moreau said.

Vector had the largest share of accepted complaints at 32.81 per cent, followed by Meridian Energy at 22.08 per cent and Mercury with 10.39 per cent.

Moreau pointed out that the number of complaints do not necessarily reflect negativity on a company.

"They can, in fact, reflect proactive customer service and effective complaint management. Customers should be informed about our service and be referred to us when appropriate," Moreau said.