A Sydney man has pleaded guilty to making sexual threats on social media, in what is being seen as a cyber-bullying test case.

Zane Alchin, 25, was charged with using a carriage service to menace, harass and cause offence.

In August last year, he wrote abusive and threatening comments on a Facebook post about 25-year-old Olivia Melville from Sydney's south.

A friend of Alchin's had taken a screenshot of Ms Melville's Tinder profile and posted it on Facebook, where it was shared thousands of times.

After that, Ms Melville received many abusive online messages from strangers.

Paloma Brierley Newton came to Ms Melville's defence on the Facebook thread and herself became the target of sexually violent comments from Alchin.

Zane Alchin said he was unaware internet trolling was a crime. ( Fairfax Media: Nick Moir )

Court documents showed that during a police interview in October last year, Alchin he told authorities he was drunk at the time and the comments did not represent who he was.

He said he was internet trolling and was unaware it was a crime, however he was remorseful for posting the comments.

Alchin had previously pleaded not guilty to the charges and a hearing was expected to begin in Downing Centre Local Court yesterday.

However, the court was told by his lawyer he had changed his plea to guilty.

Ms Melville told triple j's Hack she was glad Alchin had changed his plea after originally disputing the charges.

"I'm pretty shocked after so long him saying he's not guilty," she said.

"It will be interesting to see what the sentencing is and the weight it actually has."

He is due to be sentenced next month.

Hopes the case will bring about change for women

The case is expected to set a precedent for whether threats made on social media are punishable under existing federal law.

Ms Melville has only just spoken publicly about the abuse she suffered, for fear of provoking a further backlash.

She said she was concerned about losing her job in the performing arts and upsetting her parents, and has called on the Federal Government to fund a campaign to deter people from attacking women online.

Ms Brierley Newtown helped set up the group 'Sexual Violence Won't be Silenced', and brought the comments to the attention of authorities.

She said outside court she hoped the case would set a precedent for other women to come forward.

"Where we go from here, how we can set this to be a precedent and what that can change for women and people around the world facing harassment on mediums that haven't been recognised by the law up to this point," she said.

"By standing up and saying he's guilty of a crime he can put an end to all the backlash that this is just the internet, this isn't a crime. Well, it is a crime.

"I want to hear him say he's sorry for what he's said rather than he's sorry he got caught."