Telecom can't count and could be overcharging thousands of customers' broadband accounts.

That's the message from some customers who are complaining Telecom has been over-counting their broadband usage for several months.

The Herald has received detailed internet usage logs from two Telecom customers - one in Dunedin and the other in New Plymouth - which show over-counting broadband downloads, in one period by as much as 139 per cent. Both have raised the matter repeatedly with Telecom, but have yet to get an explanation.

"We showed Telecom five days of data as early as February 14," says director Mark Peisker of Dunedin's CueClub. "There was massive variance between our data and that reported by the Telecom usage meter. I said to them: 'Your counting has very little to do with what comes down my line to me'."

CueClub's data taken from its two internet routers shows an average of 62 per cent over-counting during a three month period - the worst month being May when Telecom counted an extra 118.24 gigabytes (GB) of usage amounting to $203.97 in overcharging.

Similarly Alister Lambert in New Plymouth first logged a fault with Telecom in January. His monitoring of his router data over the last four months reveals over-counting of around 30 per cent each month.

"This means that I get to use about 40GB of my 60GB plan before they cut it back," he said. Telecom's helpdesk told Mr Lambert a week ago they were still investigating his case, plus other cases in Palmerston North and Dunedin. "I'm getting a bit of a run-around. As I said to Telecom, if it was underestimating, I bet it would have been fixed long ago," he said.

CueClub, which is an a internet gaming centre and pool room, found that while Telecom was heavily over-counting download data (which is the majority of internet usage) it was undercounting the smaller upload stream - in one month by 39 per cent less than what CueClub was recording.

Both Mr Peisker and Mr Lambert have been advised that Alcatel-Lucent, the global telecommunications equipment corporation that runs Telecom's fixed and mobile networks, is involved in the investigation. Both were told Alcatel-Lucent had checked the network and exchange data and found it accurate, and were also looking at the interface between the network data and Telecom's billing system but could find no errors there either.

Telecom spokesperson Emma-Kate Greer said despite working closely with Mr Peisker and Mr Lambert over many months Telecom is yet to find a definitive answer.

"We are determined to get to the bottom of what is causing this and where the issue lies," she said, adding that in the meantime it is removing their caps for data use. "We monitor the network and billing files very closely and stand by the integrity of our metering system," she said. "We have more than half a million broadband customers for whom this system works smoothly."

The problem is not without precedent. In February after a public outcry Bell Canada admitted it was having problems accurately tracking internet usage for its customers.

The difficulty in knowing how widespread the problem might be in New Zealand is that few customers would know how to check their usage. Most rely on the accuracy of their internet providers' metering system in the much the same way as consumers do with their electricity or gas meters.

Telecommunications Users Association chief executive Paul Brislen said he was not surprised by the over-counting claim and had seen post on internet forums indicating other internet providers also had counting problems.

"There is no independent authority or independent assessment of these meters, so there is really no way of knowing for sure whether the 60GB of data you've used has actually been used." He said the usage meters ought to have some independent third party oversight that they were reporting accurately.

Telecommunication Dispute Resolution manager Derek Pullen said the service had received a number of complaints regarding data usage metering. He said the service has identified this as a systemic issue, and will shortly be raising this with its members.

A further complication is that while most users in New Zealand will have a monthly data cap on their internet usage, what happens when it's exceeded varies. Some plans like Mr Lambert's plans are throttled back to dial-up speed when the data cap is exceeded. Mr Lambert noted when that occurred there was no discrepancy between his download figures and Telecom's.

Other plans like CueClub's charge per gigabyte in excess of the monthly data cap - in their case at $1.50 + GST per gigabyte. Depending on their base plan others would be charged $2-$4.44 + GST per extra gigabyte used.

Questions about the accuracy of Telecom's broadband usage meter have also been raised on the Geekzone online forums since January - see here and here.

For more information on how to monitor internet see this handy guide - provided by Inga Ford who compiled the usage data for CueClub's broadband usage.