CANBERRA - By signing up to Facebook users may be unwittingly signing unfair contracts, the head of Australia's competition watchdog has warned.

Rod Sims, chairman of the Australian Competition and Consumer Commission (ACCC), said that the Cambridge Analytica scandal was good "in a sense" because it brought "how much data Facebook has" and how they can use it.

Facebook's shares have been in decline since it was revealed in March that 50 million user records had been improperly acquired by political consultancy firm Cambridge Analytica, which the firm in turn used to help elect Donald Trump as US President.

Prior to the scandal emerging, the ACCC launched a world-first probe into the Australian digital media market amid concerns that tech giants had created an uneven playing field.

"To what extent do Facebook, Google and other platform developers damage their competitors through the way they do business?" Sims said in a speech at News Corp Australia's Global Food Forum on Tuesday night.

"To what extent do users know how the data is being used? To what extent the contracts they're signed up to are fair?

"Did they know that when they connected to another website, that Facebook was gathering data on that? It's all in the terms and conditions if you want to have a look at them but I'm not sure many of us do.

"There are a whole lot of issues there. We're just starting on this exercise but it will be a fun journey."

In particular, the ACCC inquiry will focus on Facebook and Google and their "bargaining power" with publishers and the influence of their algorithms on "quality news" and journalism.