The Federal Court has ordered Optus to pay a penalty of $1.5 million for misleading customers about its National Broadband Network transition process.

Optus only gained a $750,000 benefit from its conduct, half the penalty imposed by the court, according to the Australian Competition and Consumer Commission (ACCC).

From October 2015 to March 2017, Optus told 14,000 of its customers that their internet services would be cut off in as little as 30 days unless they transitioned to an NBN plan.

However, the terms of Optus' contract did not permit disconnections in such a short timeframe.

The telco also misled customers by saying they could only sign up with Optus NBN when they, in fact, they had the option to consider competing networks like Telstra, TPG, iiNet and many others.

Optus acknowledged it was wrong in this matter. In a statement, its spokesperson said:

"Optus has written to affected customers apologising for this error and offering compensation to customers that had been disconnected without sufficient notice. "Optus has also established a revised set of migration processes for its cable customers that we believe will deliver customers the migration experience they expect."

The company had a financial incentive as it receives migration payments from NBN Co for each customer it moves to an NBN-based service.

Receiving these payments was important for Optus, which even went as far as calling them "bounty" — and formed part of their annual financial targets.

"Businesses should not make false representations which distort customers' decision making," said ACCC chairman Rod Sims.

"This is particularly important when many Australians are moving to the NBN for the first time.

Since the ACCC began investigating Optus, the telco has paid $833,000 to compensate customers who had their services disconnected.

"It is illegal for businesses to mislead their customers and create a false impression through their communications.

"Today's penalty serves as a warning to all businesses that such behaviour will be met with ACCC action."

The ACCC launched its court action in December and alleged, at the time, that Optus had misled 20,000 customers.