Optus will compensate almost 9,000 customers after failing to provide them with the fast national broadband network speeds it promised.

The telco had already admitted that some customers had not received the speeds they signed up for, and on Monday offered compensation to those who purchased NBN services between September 1, 2015 and June 30 2017.

It said it may have breached consumer law by misleading customers by promoting NBN speeds it was unable to deliver due to technical limitations on fibre to the node (FFTN) and fibre to the building (FFTB) connections.

Rival Telstra recently announced it would compensate 42,000 customers after a consumer watchdog investigation found that many customers could not receive the maximum speed listed on their plan, or even a lower-speed plan.

The Australian Competition and Consumer Commissions (ACCC) on Monday revealed that Optus customers, similarly, could not receive the maximum speed of a lower-tier plan.

ACCC chairman Rod Sims said this was a “concerning” trend throughout the industry, and one the watchdog was working to fix.

“Optus is the second major internet provider we have taken action against for selling broadband speeds they could not deliver to their customers,” Mr Sims said in a statement.

“Worryingly, many affected Optus FTTN customers could not even receive the maximum speed of a lower-tier plan.”

Optus chief executive Allen Lew did not say how many customers had been affected when he admitted the failure at the company’s half-year results in November.

But on Monday, the telco revealed that more than 8,700 customers on a number of different Optus speed plans had been affected.

The telco has provided a court-enforceable undertaking to the ACCC detailing the compensation it plans to provide to affected customers, including refunds, moving speed plans, discounted speed plans and free contract exits.

It will also be required to check within a month of connecting a customer to a new NBN speed plan that the customer is actually getting the speeds advertised and that they are paying for, the ACCC said.

“This undertaking is yet another step towards an industry standard of providing accurate information to consumers about the speeds they can achieve in real-world conditions, and ensuring that consumers get what they pay for,” Mr Sims said.

“We are continuing to investigate other retail service providers selling NBN broadband plans, and will take enforcement action if we consider that they are not delivering on their promises to customers.”

Optus will contact affected customers on or before March 2, next year.

Mr Sims urged customers to consider the options Optus was offering and to assess which best suits their needs.