Optus engaged in misleading and deceptive conduct when it used the word “unlimited” in advertising broadband plans that in fact slowed down users’ speed once the download limit was reached, the Federal Court has ruled.

The case was brought by the Australian Consumer and Competition Commission.

Justice Tony North said Optus’s advertising was misleading because it did not “sufficiently and predominantly” disclose that users’ speed would fall from about 12Mbps to 256Kbps once they had reached an allowance of 15 gigabytes per month.

Justice North says this would slow video download and teleconferencing speeds so much as to make then unusable.