The consumer watchdog says it is investigating a number of internet providers and expects to drag several to court for dudding their customers with slow broadband speeds.

Australian Competition and Consumer Commission chairman Rod Sims told a Senate estimates hearing that the watchdog was looking into claims of NBN customers paying for speeds of 100 megabits per second but only being able to connect at less than 50Mbps.

"We've certainly come across egregious examples that we are investigating to see whether or not there's been a breach of the [consumer protection] act," he said on Tuesday night.

They would either be cases of misleading conduct or breaches of consumer guarantees.

"We've got a number of investigations under way," Mr Sims said. "I'd be surprised if we don't have a couple of cases in court by the end of the year."

Telstra this month admitted that about 1 per cent, or just over 10,000, of its NBN customers were not getting the speeds they were paying for, which it discovered after reviewing the speeds customers received.

The nation's largest telecommunications provider said it would be be contacting those customers to move them to cheaper products with lower speeds Telstra could deliver, and would refund some customers.

Mr Sims said the ACCC, which earlier this year published guidelines for NBN providers to follow when making claims about internet speeds, sought to determine whether that response was adequate. It was also investigating other providers.

The watchdog is pushing ahead with plans to monitor consumers NBN speeds. Hardware will be installed in about 4000 homes to find out what connection speeds customers typically get, and how that compares with what providers advertise.

Mr Sims said his commission was seeking access to internal network information from broadband wholesaler NBN Co, which would help verify its own speed monitoring and make it easier to take action against retail providers if their claims do not stack up.

The monitoring scheme would also end the "blame game" over whether unsatisfactory speeds were due to deficiencies in NBN Co's network or because retail providers did not give customers enough capacity, Mr Sims said.

The tender for the $7 million monitoring scheme closes in late June and the ACCC will then call for volunteers to have their NBN connections monitored.