SYDNEY (Reuters) -Instagram owner Meta and other social media firms said on Tuesday they will comply with a ban on users under the age of 16, adding that they will start deactivating accounts once the law takes effect on December 10.

In parliament, Meta, TikTok owner ByteDance and Snapchat owner Snap said they continued to believe the ban would not protect young people, but they would soon reach out to owners of more than a million underage accounts to prepare them for the change.

Their comments represented a shift in the social media industry's response to the law, which is being watched by lawmakers around the world as concern grows about youth mental health. Under the Australian law, platforms must take "reasonable steps" to block users aged less than 16 or face a fine of up to A$49.5 million ($32.5 million).

The platforms previously argued that the ban would drive young people to more dangerous corners of the internet that are poorly monitored, as well as deprive young people of social contact. They also said that implementation would be unnecessarily complex. Snap and Google-owned YouTube have also argued they aren't social media companies.

"We don't agree, but we accept and we will abide by the law," said Jennifer Stout, Snap's senior vice president of global policy and platform operations, via a video link.

Ella Woods-Joyce, TikTok's public policy lead for Australia, reiterated the Chinese-owned platform's opposition to the ban but said "TikTok will comply with the law and meet its obligations".

"We are on track to meet our compliance," she said.

Mia Garlick, Meta's policy director for Australia and New Zealand, said the company would soon approach holders of accounts confirmed to be under 16 - about 450,000 across Instagram and Facebook - to give them a choice between deleting their photos and other data or offering to store it until they turned 16.

TikTok, which says it has 200,000 under-16 accounts in Australia, and Snap, which says it has 440,000 under-16 accounts, said they would take similar steps. The companies added that they would use automated behaviour-tracking software to determine if an account holder claiming to be over 16 was underage.

"Where we identify someone that is saying they're 25 but the behaviors would indicate that they're below the age of 16, from December 10th we will have those accounts deactivated," Woods-Joyce said.

For users incorrectly deemed to be under 16, Meta and TikTok said they would refer them to a third-party age-estimation tool. Snap said it was still working on a solution for users who believed they were incorrectly blocked.