Telstra has offered to refund 42,000 customers for slow national broadband network speeds, following an investigation by the consumer watchdog.

The telco will offer remedies to those who purchased internet services through both Telstra and Belong brands between September 2015 and November this year after admitting it breached consumer law by promising NBN speeds that it was not capable of delivering.

“All businesses have a responsibility to ensure that claims about the performance of their products or services are accurate,” the Australian Competition and Consumer Commission chairman, Rod Sims, said.

One of the affected services was a “Super Fast Speed Boost” plan, which advertised maximum download speeds of up to 100 megabits per second and maximum upload speeds of up to 40 Mbps (100/40 Mbps).

In reality, many of Telstra’s fibre to the node and fibre to the building customers were not even capable of receiving the maximum speeds of a lower-speed plan, Sims said, adding: “In essence, people were paying more to get higher speeds that they just weren’t able to get.”

Telstra will offer affected customers a range of remedies, including refunds, the option to change speed plans, and exit from contracts without paying a fee.

Telstra went to the ACCC to notify it of issues relating to some, but not all, of the affected customers, which the ACCC investigation subsequently uncovered.

“We are pleased that Telstra proactively reported this serious problem to the ACCC and has cooperated in creating a remediation plan for affected customers,” Sims said.

“However, we are mindful this is not just a Telstra problem; it is an industry problem where consumers are often not getting the speeds they are paying for.

“We will continue to investigate other retail service providers selling broadband plans over the NBN and take enforcement action where appropriate.”

Telstra has made a commitment to check the attainable speeds of all new service connections within four weeks. If speeds do not match the advertised offer, it will offer the customer a remedy.