A billing error led Maine Natural Gas to significantly undercharge some of its largest customers, the Public Utilities Commission has discovered, and the agency said Wednesday that it’s opening an investigation into the matter.

The PUC wants to determine, among other things, what led to the underbilling, whether the error could have been detected and what the effect may be on other customers.

Maine Natural Gas has more than 4,200 customers in Bath, West Bath, Brunswick, Topsham, Windham, Gorham, Bowdoin, Freeport, Pownal, Hallowell and Augusta. There are roughly 43,000 natural gas customers in Maine, according to federal data.

After a consumer complaint, the agency found that the billing errors dated back to 2002 and extended to 2016. It found 11 large customers were involved, but the names are protected from public disclosure, the PUC said.

The error was was traced to billing units assigned to the meter reading by the gas company’s billing office. Meters could be fitted with gauges that measured gas in million cubic feet (Mcf) or hundred cubic feet (Ccf). The company assumed all the meters had been set at hundred cubic feet, but when it conducted an inspection, 11 accounts were found to have been underbilled.

When it learned of the problem, Maine Natural Gas issued make-up bills, seeking to recover the unbilled amounts going back 12 months. But with so much time having gone by, the PUC is looking into whether rates were affected, whether other customers paid too much while the large customers were undercharged.

“It remains an open question whether further adjustments to cost of gas or other rates may be warranted to correct inequities to other customers,” the commission said in opening the case.

Maine Natural Gas is a subsidiary of Avangrid, the parent company of Central Maine Power.

An initial PUC conference on the case has been set for Sept. 21 at its offices in Hallowell.