Tens of thousands of NBN customers who experience slow connection speeds have been urged to contact their telco provider as they may be eligible for a refund.

The ACCC said on Wednesday NBN customers may be entitled to a refund after internet providers admitted to making false or misleading claims about speeds.

Telstra, Optus, TPG, iiNet, Internode, Dodo, iPrimus and Commander have all admitted to advertising and selling NBN plans with unattainable connection speeds.

Tens of thousands of NBN customers who experience slow connection speeds have been urged to contact their telco provider as they may be eligible for a refund

Due to the limitations of FTTN and FTTB technologies, consumers could never experience the advertised download speeds of up to 100 megabits per second.

The consumer watchdog said they had been negotiating with telcos over the last 15 months and accepted undertaking from each that they would contact customers.

ACCC Acting Chair Mick Keogh said a large proportion, two in three affected consumers, have not responded to letters or emails from their internet provider.

'They may be eligible for refunds, some in the hundreds of dollars,' Mr Keogh said.

'The ACCC is urging NBN customers to contact their NBN retailer if they have received a letter or email offer of a remedy, or think they might be entitled to a remedy.'

The watchdog said customers who have recently signed up to a new NBN plan may also be eligible for a refund if the telco had advertised maximum connection speeds.

Telstra, Optus, TPG, iiNet, Internode, Dodo, iPrimus and Commander have all admitted to advertising and selling NBN plans with unattainable connection speeds

Within four weeks, the telco must check their speeds and if the they are below the advertised speeds for the plan the consumer chose, they must offer remedy options.

The ACCC said the amount of any refund depends on the price someone pays for a maximum download speed and the price of the plan that can actually deliver it.

For example, if you pay $100 a month for a maximum download speed of 100 Mbps, but only receive 37 Mbps and a $70 a month plan delivers 50 Mbps, then the refund would be $30 for very month already paid.

Mr Keogh said telcos are expected to provide consumers with accurate information up front about the internet speeds they can expect to experience and then deliver.

'Our message to RSPs (telcos) is that if you advertise a particular connection speed and customers cannot experience that speed, you risk breaching the Australian Consumer Law,' he said.