Less than half of those able to join the National Broadband Network have done so and those who have joined are accessing it at some of the lowest speeds available, a draft report released on Monday by the Australian Competition and Consumer Commission shows.

The NBN is available to 6.2 million Australian premises but only 3 million of those have migrated to an NBN plan, according to the ACCC.

According to the NBN Co, this number is misrepresentative of the uptake percentage, with 75 per cent of those connected to the NBN migrating to an NBN plan after 18 months.

The ACCC report said that another of the reasons behind the gap was that some providers were being incentivised to keep customers on the older network where they are able to earn better margins.