Despite widespread condemnation, the maverick One Nation MP has refused to apologise for a tweet he posted on March 30 that Mr Greenwich described as "defamatory and homophobic".
"He followed that up with media commentary that doubled down on his attack," Mr Greenwich said in a statement on Monday.
The comments were in response to Mr Greenwich calling Mr Latham "a disgusting human being" after LGBTQIA+ protesters were attacked outside a candidates' forum including Mr Latham as a speaker, during the NSW election campaign.
When pressed for an apology, Mr Latham tweeted "never apologise, never explain".
Mr Greenwich says unless Mr Latham apologises and makes amends he will begin defamation proceedings.
The independent MP has also lodged a complaint of homosexual vilification with the Anti-Discrimination Board of NSW, and made a police complaint against Mr Latham for using a carriage service to menace, harass or offend.
"Enough is enough".
"I've been openly gay in public life for 15 years and in NSW Parliament for over 10 years, and never have I experienced such a homophobic, sexualised attack that exposed me to contempt, ridicule and extreme abuse, based on my sexuality,."
Mr Greenwich says he is also taking action for young and old LGBTQIA+ people who face regular abuse.
His legal team will also approach Twitter about its provision of a platform that "freely allows extreme homophobia, transphobia, racism and religious bigotry".
The tweet caused a schism between the NSW leader and federal leader Pauline Hanson who said the comments were "disgusting" and called on him to apologise.
She said he ignored her calls and texts.
Premier Chris Minns condemned the comments by Mr Latham, saying they were "terrible" and would "unleash ghouls" on the MP.
Mr Latham was re-elected for an eight year term at the March poll but the legal action could impact his ability to serve if he is charged with using a carriage service to menace, harass or cause offence.
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