ActewAGL chief executive Michael Costello has unreservedly apologised for the problems, saying he failed to foresee the scale of the issues and to notify customers ahead of time about the changes and potential effects.

The Canberra Times has received numerous letters, particularly in the past few months, complaining of potential "over-charging" due to bill estimations by the ACT's major gas distributor and retailer since last year.

Figures from ActewAGL also show the utility received some 2600 direct complaints during the September quarter last year, or about 900 more complaints than usual.

Mr Costello said the problems stemmed from a change to gas retail market procedures by the Australian Energy Market Operator, which reduced the amount of time allowed between a meter being read and a bill being processed from five to two days.

He said the billing process was a complex pipeline that stretched from meter readers to distributors to the market operator and finally to retailers and if meters had not been read and verified before the two day deadline, national rules demanded an estimate be used to calculate bills.

"We think we're at the stage now, and have been for some months, where we're back to where we are in the normal state of things, [but] we're not being complacent about this I can assure," he said.

"There's nothing I can do except say I am deeply apologetic, we accept responsibility for it, we are not trying to evade that responsibility.

"We are putting every effort in to ensure that this problem stays away, because it is away now, but we still have the hangover from the last six months of last year."

Despite ActewAGL being given 12 months to adjust, and the utility setting up an internal project team to complete the changes, Mr Costello said he did not fully foresee the impacts the changes would have.

But he said he would personally ensure that "not a single customer will be charged a single dollar for any gas they did not consume" and that any customers with concerns should contact ActewAGL.

He said the community's trust was one of the biggest assets the utility had, and it would not do anything to damage that trust.

While bills are usually based on actual meter readings in more than 90 per cent of cases, about three to five per cent of bills are based on "estimates" each quarter and are labelled as such on the bill.

But Mr Costello said ActewAGL had only anticipated up to 10 per cent of bills would need to be estimating during the first quarter under the new rules, from July to September last year.

He said the actual number of estimations rose to 25 per cent of its 130,000 customers across the ACT and New South Wales - or about 32,500 individual customers facing "adjustments" from the September quarter.

In that quarter alone, over-estimations totalled $326,000 and under-estimations reached $401,000, while the internal changes cost ActewAGL about $350,000.

Mr Costello said that by December last year, the number of bills based on estimation had fallen to about five per cent, and in the March quarter to about three per cent, but that "adjustments" to bills were still being made.

Mr Costello said that all customers affected by an "adjustment" - either an over or under estimation - would have an equivalent adjustment made to their next quarterly bill, guaranteeing they would not be left out of pocket overall.

Asked why ActewAGL did not alert its customers about the change ahead of time, to avoid potentially months of uncertainty for customers, Mr Costello said "we should have".

"We did not think the problem would be anything like as large as it was, it never crossed our mind it would be anything like 25 per cent of our customers," he said.

"We expected some minor increase [in estimates], but not the problem we had."