The Canberra Times has been sent a bill which charged an Ainslie property owner half a million dollars for three months' of water use.

The family was charged $563,980 for using 200 million litres of water - the equivalent of draining and re-filling two Olympic-sized swimming pools on each day of the billing period.

The bill contained some worrying words - "do not pay" - which meant the property owner had already set up direct debit.

"I was contemplating a very sarcastic letter asking (ActewAGL) to leave me the loose change after selling my house from under me," said the man who received the bill.

"But (ActewAGL) phoned the following day to apologise and say that a new invoice was on the way.

"Indeed it was, and there was a credit of $563,980 and all was well."

The man, now in his late 50s, received the bill in 2007 and it probably became quite well-known within ActewAGL.

ActewAGL executive Jane Corkhill said she remembered the case, even though at the time she had not risen to the rank she holds at the moment as the utility company's customer service manager.

She said the bill prompted changes to the way customers were billed. By the time this mistake happened a new review process - with two separate validation hurdles for bills to pass through - had already been put in place.

But as Ms Corkhill explained, there was a flaw in the system.

The checking system was strong at picking up whether there were any unexplained increases or decreases in charges, compared to previous bills a property had received, and stopping dodgy bills before they were posted.

But a problem arose when a new meter was installed at a property, as was the case with the man charged half a million dollars.

The system had nothing to compare that first bill with.

"We were very embarrassed and very quick to talk to the customer," Ms Corkhill said.

The customer said he was happy with the way it was handled by the company and has treated it as a funny story to tell others since.

ActewAGL ensured another checking process happened, via a computer program, when the bills were printed.

But the utility company still comes under fire for controversial bills.

This month it turned a family's gas off and billed them $3300 over a metering mistake the power company made, only to backflip after being contacted by this news outlet but not before an injured woman was forced to couch surf for a week because there was no heating in her family's apartment.

Ms Corkhill said she was aware of just two crossed meter dramas in the past year - the meter reading is done by a separate company - including this case.

In a separate case involving a Canberra GP disputing a $25,000 home energy bill for 12 months use which was partly still in progress, the ACT Civil and Administrative Tribunal raised concerns there was no independent body testing the accuracy of meters in the ACT.