Telstra's attempt to upgrade 770,000 NBN customers to higher speed plans for free has partially backfired.

Reports say up to 180,000 services will likely need to be downgraded again to a lower NBN speed tier, after an issue dubbed the ‘FTTN/B curse’ resurfaces.

“Telstra’s failure to carry out the speed checks meant that customers were not informed on whether they were getting the maximum speeds promised under the upgraded NBN plans,” the ACCC said in a statement on Thursday.

“It also meant they were not offered options to address slow speeds, such as exiting their contract or receiving a refund.

“Telstra has since committed to contacting all affected customers and refunding those who have been paying for the higher speeds but not receiving them.

“It will also proactively move consumers to a lower speed NBN plan if they are not receiving any benefit from being on a higher speed tier NBN plan.”

Telstra self-reported the issue and has begun remediation efforts. The company has issued a statement acknowledging its error.

“In November 2017, we promised that we would check the speeds customers were receiving four weeks after they set up a new NBN service with Telstra, and if the speed was below what was advertised we would let them know and work with the customer to fix the issue,” a Telstra spokesperson said.

 “We found that some customers who had moved from one NBN plan to another were not hearing from us on the speeds they were receiving and were paying for speeds they could not get. 

 “We let the ACCC know and started contacting customers as soon as we found out.

“We’re disappointed that we’ve let some customers down and we know this isn’t good enough.”