Telstra will pay $15 million after action was taken by consumer watchdog, the ACCC in the Federal Court.
The penalty was agreed between Telstra and the watchdog, and approved by Justice Mark Moshinsky on Friday.
Optus agreed to pay $13.5 million while TPG will pay $5 million.
Each of the three providers admitted liability for contraventions of the Australian Consumer Law, Justice Moshinsky said.
Three separate cases were brought by the Australian Competition and Consumer Commission over representations made to residential customers about broadband internet services using the NBN.
The judge said each of the representations differed, but broadly involved customers being told their maximum attainable speeds would be tested within a reasonable time.
In a majority of cases that occurred, but some customers did not have speeds tested and ended up subscribing to a plan for speeds they couldn't achieve.
Customers weren't notified or given the option to move to a different plan, Justice Moshinsky said.
Reasons for the orders will be handed down at a later date.
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