An allegedly defamatory tweet by former One Nation leader Mark Latham about independent Sydney MP Alex Greenwich "went as low as possible", a court has heard.

Mr Greenwich is suing the NSW upper house MP in the Federal Court, alleging the tweet in late March 2023 exposed him to hatred, contempt and ridicule from the public and "gravely injured" his personal and professional reputation.

A five-day hearing before Justice David O'Callaghan began today in Sydney.

Mr Latham's publication was in reply to comments by the independent MP in a news article about LGBT protesters being targeted in a violent confrontation outside a church in Sydney's west, where Mr Latham was speaking.

Mr Greenwich referred to Mr Latham as a "disgusting human being".

Mr Latham's response on X, formerly Twitter, which the ABC has chosen not to publish in full, began with "Disgusting? …" before describing sexual activity in explicit terms.

Mr Greenwich's counsel Matthew Collins QC said the two politicians sat "at different points in the political spectrum" in a state where politics was practised "pretty robustly".

Mr Latham focused on Mr Greenwich's 'sexuality', court told

During an opening address, the barrister showed the court a series of social media posts by Mr Latham in which he used photographs of the independent MP in the months leading up to the church incident.

"Mr Latham posted various matters on social media targeting Mr Greenwich, always focusing on his sexuality," Dr Collins said.

"They were certainly ugly, but they at least had a tenuous connection to matters which were going on in the city — the World Pride festival and matters of policy by reference to family policy."

But things "escalated rather seriously", Dr Collins told the judge.

In reference to the church incident, Mr Latham complained on social media about "the new rainbow fascism" and Dr Collins said in an interview he also accused Mr Greenwich of being an "instigator" of the situation.

By providing quotes to The Sydney Morning Herald about the church incident, Mr Greenwich was "engaging bonafide in a public debate arising out of a notorious incident that was a matter of considerable public interest", according to Dr Collins.

"Mr Latham, rather than engaging in that debate, went as low as possible in the … tweet and then the conduct which followed."

Death threats and reputational harm

Dr Collins said the reaction included death threats and "unbelievable epithets" about Mr Greenwich's sexuality.

Those stereotypes were that gay men were "somehow depraved" because of Mr Latham's presumption about sexual activities, and that they "can't be trusted with children", the barrister said.

The later-deleted tweet by Mr Latham, which Dr Collins described as containing "revolting words", was posted at a time when some 66,000 accounts followed his profile.

Mr Greenwich alleges his reputation was harmed by four imputations, or meanings, both in the tweet itself and in subsequent comments Mr Latham provided to The Daily Telegraph.

The alleged imputations of the tweet include that Mr Greenwich "engages in disgusting sexual activities" and that he "is not a fit and proper person to be a member of the NSW parliament because he engages in disgusting sexual activities".

The alleged imputations of the comments in The Daily Telegraph include that Mr Greenwich "is a disgusting human being who goes to schools to groom children to become homosexual" and is "not a fit and proper person to be a member of the NSW parliament".

In his defence, Mr Latham has argued that he was publicly attacked before posting the tweet and raised defences of qualified privilege, public interest and honest opinion.

He claimed the articles published in the fallout of the incident and responses from politicians "enhanced the reputation of Greenwich".