The competition watchdog has warned Australia's largest four telcos they could find themselves in court for failing to deliver on their NBN speed promises.

Telstra, Optus, TPG and Vocus have been singled out by Australian Competition and Consumer Commission chairman Rod Sims for potential legal action by the end of the year if they are found to have misled consumers.

Mr Sims told AM the telco giants have now been instructed to refer to "typical" rather than "up to" speeds, especially in peak periods, when advertising NBN products.

"We've got a broadband monitoring regime in place so we can tell consumers what the speeds are and also check whether those providers are telling the truth," Mr Sims said.

"We'll have a few cases in court by the end of the year. We want to make sure that we solve this problem."

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Mr Sims said, while all internet providers are subject to consumer law, the regulator would be targeting "the big ones".

"The main focus will be on Telstra, Optus, Vocus and TPG. They have been put on notice and some of them know that they are subject to investigation by us," he said.

"We have the reputation for following through on these things and we will."

ACCC not going far enough: advocacy group

While Mr Sims' language about questionable NBN speed promises has ramped up, the ACCC has not taken action against a telco in the past five years.

The threat of legal action comes amid growing concerns that the NBN has not been funded to provide high speed broadband to Australians.

Internet Australia, a not-for-profit organisation representing internet users, supports the ACCC's monitoring program but maintains the NBN has been "turned into a dud".

Executive director Laurie Patton said the regulator's surveillance will not be enough to solve the overriding problems plaguing the NBN rollout.

"The ACCC scheme will simply confirm what we already know. Consumers are disappointed with their internet speeds," Mr Patton said.

"This is because the NBN Co model is fundamentally flawed. Relying on ageing copper wires means NBN Co simply cannot deliver very fast broadband to millions of customers.

"On top of this, their wholesale charging discourages retailers from offering faster speeds."

As part of its monitoring, the ACCC has called on Australians to volunteer for a program to measure and compare broadband speeds across the county.

In a statement, a spokesperson said Telstra would cooperate with the ACCC's monitoring regime.

"We will continue to take an industry lead to ensure our customers receive the service and experience they expect on the NBN armed with all the information they need to make an informed choice," it read.

Optus and TPG have been contacted for comment. Vocus declined to comment.

Follow Peter Ryan on Twitter @peter_f_ryan.