The billing and customer service practices of Hydro One will be the subject of an investigation by the Ontario ombudsman, following a growing number of complaints from its customers.

Ombudsman Andre Marin announced the details of the investigation at a news conference on Tuesday morning.

"This investigation is the results of years of behind-the-scenes efforts by my staff to resolve hundreds of complaints, one case at a time," he told reporters at Queen’s Park.

"We’ve helped many people sort out egregious errors and baffling bills. We’ve worked with senior Hydro One officials to ensure they credit people who are overbilled and don’t cut off power to people in need."

But the ombudsman said the complaints continue to mount.

Marin said that his office has already dealt with twice the number of Hydro One-related complaints only 10 months into the fiscal year, compared to the previous year.

“The stories we’re hearing will be familiar to many of you in the media — stories of huge unexplained, catch-up bills, multiple bills or estimated bills with no rhyme or reason,” he said.

"And when customers try to get answers from Hydro One, they are stymied, just as my office has often been stymied when we intervened."

These complaints have prompted Marin to arrange for the special ombudsman response team to investigate the problems being reported.

"In order to help the greatest number of people in the shortest amount of time, we will focus on the serious billing and communications issues that customers have raised about Hydro One and we will complete it within nine months," he said.

Hydro One is owned by the province of Ontario. It operates most of the electricity transmission system in Ontario and provides power to more than one million customers across the province.

Tiziana Baccega Rosa, spokeswoman for Hydro One, told CBC News in a telephone interview that the company respects the decision of the ombudsman’s office, and will work “openly and constructively” with its staff during the investigation.

Baccega Rosa said Hydro One acknowledges that its level of service hasn’t been acceptable to customers in some cases, and that the company has had problems with its customer information system and call centre.

She said the kind of billing problems the ombudsman was talking about are specific to five per cent of Hydro One’s customers.

About 3.2 per cent of customers have been getting multiple, estimated bills and another two per cent of customers have gone 90 days or more without receiving bills — in both cases, Baccega Rosa said, Hydro One is working through the problems.