Ms Garrett's position within the Labor Party has ignited open conflict between two of the state's most militant unions, the CFMEU and the United Firefighters Union.

Both have fired off letters accusing leaders in the other union of selling out the labour movement.  

The building union is flexing its muscle on behalf of its long-time ally Ms Garrett, saying it is "horrified" by an online campaign being run against the former minister.

The United Firefighters Union and its hardline leader Peter Marshall have been lobbying state Labor MPs, the Premier and even ALP federal leader Bill Shorten to dump a proposed deal between the party's factions that would see Ms Garrett make a comeback in a safe seat in Victoria’s upper house.

The Brunswick MP, who will leave her seat at November’s election, and the union have a history of conflict going back to 2015 when Ms Garrett was Emergency Services Minister.

Now the building union alleges that an anonymous Twitter account, set up to target Ms Garrett and two other Labor women, sent a message referring to the “murder” of the former minister.

The Age does not suggest that Mr Marshall or the UFU had any involvement with sending the tweet or setting up the account.

More than two years after Premier Daniel Andrews intervened in favour of the UFU in its dispute with Ms Garrett in an effort to end the industrial stand-off, the damaging issue shows no sign of going away.

In a letter sent to the Premier and to all state Labor MPs, the CFMEU’s Victorian Branch Assistant Secretary Shaun Reardon has accused the firefighters union of running a campaign against Ms Garrett.

“The UFU have been involved in a somewhat bitter dispute in relation to an Enterprise Bargaining Agreement for their members,” Mr Reardon wrote.

“Since Ms Garrett resigned from her position of Minister of Emergency Services, (more than two years ago), I believe she has been 'scapegoated' by certain party members, assaulted near her family home, and is the victim of a misogynistic social media campaign.

“This campaign saw a tweet recently joking about the murder of Jane Garrett on the very same day that a memorial was being held for the sickening death of young woman Eurydice Dixon.”

Mr Reardon called on the Premier to “stand strong against the campaign being waged against Ms Garrett".

“As a party member, I am horrified that another party member, and a union leader at that, would threaten to campaign against the return of an Andrews [led] Labor Government,” the building union leader wrote.

“I implore you to stand strong on party matters with any undertone of misogyny or workplace bullying and to continue your outstanding work in relation to violence against women.”

The Age understands the state party's administration committee decided last month to investigate the complaint.

The UFU branch committee of management responded to Mr Reardon's accusations with an angry letter of its own on Tuesday.

"For Shaun Reardon to somehow link the UFU or Peter Marshall, however tenuous, to the vile alleged death threat to Ms Garrett is quite frankly abhorrent," the committee said.

It accused Mr Reardon of twisting the facts about an physical attack on Ms Garrett in August 2016 to imply the UFU was involved.

"Our understanding of the facts is that Ms Garrett was physically assaulted by a woman suffering severe mental health issues and [who] has absolutely no link to the UFU or professional firefighters," it reads.

Mr Marshall, who has been contacted for comment, has said Mr Andrews should tear up a proposed deal that would see Ms Garrett take the place in the upper house of Eastern Victoria MP Daniel Mulino, who would take up the federal seat of Maribyrnong held by Mr Shorten who in turn would move to the newly created electorate of Fraser.

The UFU leader says Ms Garrett's "union busting" during her time as a minister means she should not be allowed to be a Labor MP.

"On behalf of the membership of the UFU, I ask you to seriously consider the suitability of Ms Garrett for preselection of the Labor Party and reintroducing Ms Garrett into Victorian politics after her failure to encompass Labor values during her time as Minister," Mr Marshall wrote to the Premier.