Large companies that fail to remove false and misleading comments from their brands' Facebook pages within 24 hours face potential court action, the competition watchdog has warned.

Brands are required to monitor comments left by the public to ensure they comply with advertising guidelines and Australian consumer law, following a ruling by the advertising industry's regulator last week that everything that appears on a brand's Facebook page is advertising.

Now the Australian Competition and Consumer Commission has lent its voice to the Advertising Standards Board ruling, which has thrown the marketing industry into turmoil as companies face the prospect of being penalised for what the public is posting on their sites.

An ACCC commissioner, Sarah Court, said she expected big brands that use Facebook to promote themselves to take down comments within a day or less.

''If you are a big corporate player with lots of resources that's putting a lot of effort into social media then it wouldn't have to be too long. Perhaps 24 hours or less,'' Ms Court said.

The ACCC would take a dim view of companies that left posts that they knew contained incorrect information about a product or service, whether it was their own or a competitor's, or if the company ''engaged in banter'' with the person posting the message, she said.

''A court would have to be satisfied that a big company had somehow become aware and ignored these false testimonials … If you knew about them and they worked to your advantage and left them there, then I don't think you could say: 'It isn't our fault and therefore it isn't our responsibility'.''

Last week VB was found responsible for abusive comments about gays and women that were posted on its Facebook page and was therefore in breach of the industry's self-regulatory codes on taste, decency and ethics.

And because the comments were in response to a question that VB's marketing team had posted (how to make a great Australia Day BBQ) the board ruled they constituted advertising. VB's owner, Fosters, has since introduced new rules that include twice-daily monitoring of user comments.

Advertisers are now meeting to draw up new guidelines.

Chief executive of industry body the Communications Council, Margaret Zabel, said: ''We have initiated discussions with the Australian Association of National Advertisers and the board to establish what constitutes a reasonable time frame [to remove posts] so that we have clarity and certainty on the issue.''