Mark Latham faces another hefty legal bill after he was found to have vilified and sexually harassed a fellow member of parliament.

The former federal Labor leader and One Nation MP has been ordered to pay Sydney MP Alex Greenwich the maximum penalty of $100,000 after a tweet directed at him was found to incite hatred based on his sexuality.

Latham's tweet was previously found by the Federal Court to be defamatory and he was ordered to pay Greenwich $140,000 in legal costs.

'The primary tweet was so obnoxious that, in the main, media sources did not repeat it,' the NSW Civil and Administrative Tribunal said in its decision on Thursday.

'This reticence by media outlets, usually hungry to print the comments of high-profile politicians, also bespeaks a 'line having been crossed' from acceptable commentary to unacceptable vilification.'

Latham's tweet came in response to a news article quoting Greenwich describing him as a 'disgusting human being'.

The online sparring match between the two politicians followed violent protests outside a church in Sydney's southwest, where Latham was giving a pre-election speech in March 2023.

About 250 mostly male counter-protesters violently attacked police and 15 LGBTQI protesters.

The tribunal imposed the maximum penalty because of the extreme abuse Greenwich received and the intense impacts he felt.

Staffers for Greenwich testified his electorate office suddenly received a barrage of hate mail after the incident, including messages 'talking of throwing homosexuals off the cliff'.

A psychologist testified that Greenwich met the criteria for a diagnosis of post-traumatic stress disorder as a result of the attacks against him.

'Mr Latham's comments sought to reduce my identity to a sex act and then imply I am a danger to children,' Greenwich said in evidence put to the tribunal.

'Mr Latham's comments have impacted my ability to attend large gatherings out of fear that people will ask me questions about Mr Latham or engage in violence against me.'

The tribunal did not order Latham to produce an apology, saying it would be pointless given it would likely not be genuine.

In a further rebuke from the tribunal, Latham was accused of showing a lack of respect for the integrity of the proceedings, which contributed to him being ordered to pay Greenwich's legal costs.

'Some of these comments about the tribunal and the proceedings may have 'fed' continuation of the barrage of responses online,' the tribunal said.

Latham showed no signs of contrition after the judgment, posting to X it was a 'woke, leftwing political judgement' and described the proceedings as a 'Mad Hatter's Tea Party'.

He said one of the tribunal members was a prominent transgender activist who was hostile towards him and he would appeal the decision.

The upper house MP had argued he was not responsible for the vilification inflicted by others on Greenwich because their prejudice pre-dated the feud between the pair.

The tribunal rejected the argument, saying the 'avalanche' of abuse Greenwich received after the incident was much greater than he had previously received, including during debate over marriage equality.

'A respondent cannot escape unlawful vilification simply because they enflamed the emotions of a group of people known to have prejudiced views,' the tribunal said.

Greenwich welcomed the decision and said the result was a landmark ruling for LGBTQIA+ rights in the state.

'This decision sends a clear message: public figures are not above the law, and online platforms are not a space for unlawful vilification.,' the member for Sydney said in a statement.

'The tribunal has made clear that the law does not require anyone to endure unlawful vilification or sexual harassment.'

Latham was ordered to remove any material from his social media that vilifies Greenwich on the basis of his homosexuality and to refrain from making any more unlawfully vilifying statements.