Similarly Alister Lambert in New Plymouth first logged a fault with Telecom in January.
His monitoring of his router data over four months revealed 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 said yesterday that the amount of compensation would depend on the customer's circumstances.
If speeds were slowed to dial-up after a data cap was exceeded, then compensation will given for the number of days the internet was slowed down for.
If a customer was charged for additional data, then the compensation will equal the charges billed to an account during the period of time the data was incorrectly calculated.
Telecom said affected customers should receive a letter by August 12, or a note with their next bill detailing the compensation.